sharness: add customized vim syntax w/ heredoc support

problem: new heredoc supporting commands don't highlight nicely

solution: add them to the syntax file

To use this with vim-plugged, add this to vimrc:

```vim
Plug 'flux-framework/flux-core', { 'rtp': 't/sharness/vim' }
```

t/sharness/vim/ftdetect/sharness.vim

@@ -0,0 +1 @@
+au BufRead,BufNewFile *.t set filetype=sh.sharness

t/sharness/vim/indent/sharness.vim

@@ -0,0 +1,3 @@
+let b:sh_indent_options = { 'continuation-line': 0 }
+
+runtime! indent/sh.vim

t/sharness/vim/syntax/sharness.vim

@@ -0,0 +1,50 @@
+let b:is_bash=1
+runtime! syntax/sh.vim
+
+syn keyword shsStatement test_done
+syn keyword shsStatement test_set_editor test_set_index_version test_decode_color lf_to_nul nul_to_q q_to_nul q_to_cr q_to_tab qz_to_tab_space append_cr remove_cr generate_zero_bytes sane_unset test_tick test_pause debug test_commit test_merge test_commit_bulk test_chmod test_modebits test_unconfig test_config test_config_global write_script test_unset_prereq test_set_prereq test_have_prereq test_declared_prereq test_verify_prereq test_external test_external_without_stderr test_path_is_file test_path_is_dir test_path_exists test_dir_is_empty test_file_not_empty test_path_is_missing test_line_count test_file_size list_contains test_must_fail_acceptable test_must_fail test_might_fail test_expect_code test_i18ncmp test_i18ngrep verbose test_must_be_empty test_cmp_rev test_cmp_fspath test_seq test_when_finished test_atexit test_create_repo test_ln_s_add test_write_lines perl test_bool_env test_skip_or_die mingw_test_cmp test_env test_match_signal test_copy_bytes nongit depacketize hex2oct test_set_hash test_detect_hash test_oid_init test_oid_cache test_oid test_oid_to_path test_set_port test_bitmap_traversal test_path_is_hidden test_subcommand
+syn keyword shsStatement test_cmp test_cmp_config test_cmp_bin packetize
+
+syn region shsTest fold start="\<test_expect_\w\+\(_hd\)\?\>" end="$" contains=shsTestTitle
+syn region shsTest fold start="\<test_expect_\w\+\(_hd\)\?\>\s\+\<[A-Z_,]\+\>" end="$" contains=shsPrereq
+syn region shsTest fold start="\<test_lazy_prereq\>\s\+\<[A-Z_,]\+\>" end="$" contains=shsPrereqLazy
+
+syn keyword shsTestStatement contained containedin=shsTest test_expect_success test_expect_failure test_expect_unstable test_lazy_prereq test_expect_success_hd test_expect_failure_hd test_expect_unstable_hd
+
+syn region shsTestTitle contained start=' 'hs=s+1 end=' 'me=e-1 nextgroup=shsTestBody contains=shSingleQuote,shDoubleQuote
+
+" multiple line body
+syn region shsTestBody contained transparent excludenl matchgroup=shQuote start=+ '\\\?$+hs=s+1,rs=e end=+'$+ contains=@shSubShList
+syn region shsTestBody contained transparent excludenl matchgroup=shQuote start=+ "\\\?$+hs=s+1,rs=e end=+"$+ contains=@shSubShList
+syn region shsTestBody contained transparent excludenl matchgroup=shHDBody start=+ <<-\?'\z(\w\+\)'$+hs=e,rs=e end=+^\s*\z2$+ contains=@shSubShList
+
+" single line body
+syn region shsTestBody contained oneline transparent excludenl keepend matchgroup=shQuote start=+ '+hs=s+1 end=+'$+ contains=@shSubShList
+syn region shsTestBody contained oneline transparent excludenl keepend matchgroup=shQuote start=+ "+hs=s+1 end=+"$+ contains=@shSubShList
+
+" heredoc quotes
+syn region shHereDoc matchgroup=shRedir start="<<\s*\\\"\z(.\{-1,\}\)\\\"" matchgroup=shRedir end="^\z1\s*$"
+syn region shHereDoc matchgroup=shRedir start="<<-\s*\\\"\z(.\{-1,\}\)\\\"" matchgroup=shRedir end="^\s*\z1\s*$"
+syn region shHereDoc matchgroup=shRedir start="<<\s*\\\\\z([^ \t|>]\+\)" matchgroup=shRedir end="^\z1\s*$"
+syn region shHereDoc matchgroup=shRedir start="<<-\s*\\\\\z([^ \t|>]\+\)" matchgroup=shRedir end="^\s*\z1\s*$"
+syn region shHereDoc matchgroup=shRedir start="<<\s*\\\\\_$\_s*\z([^ \t|>]\+\)" matchgroup=shRedir end="^\z1\s*$" contains=@shDblQuoteList
+syn region shHereDoc matchgroup=shRedir start="<<-\s*\\\\\_$\_s*\z([^ \t|>]\+\)" matchgroup=shRedir end="^\s*\z1\s*$" contains=@shDblQuoteList
+syn region shHereDoc matchgroup=shRedir start="<<\s*\\\\\_$\_s*\\\\\z([^ \t|>]\+\)" matchgroup=shRedir end="^\z1\s*$"
+syn region shHereDoc matchgroup=shRedir start="<<-\s*\\\\\_$\_s*\\\\\z([^ \t|>]\+\)" matchgroup=shRedir end="^\s*\z1\s*$"
+syn region shHereDoc matchgroup=shRedir start="<<\s*\\\\\_$\_s*'\z(.\{-1,\}\)'" matchgroup=shRedir end="^\z1\s*$"
+syn region shHereDoc matchgroup=shRedir start="<<-\s*\\\\\_$\_s*'\z(.\{-1,\}\)'" matchgroup=shRedir end="^\s*\z1\s*$"
+syn region shHereDoc matchgroup=shRedir start="<<\s*\\\\\_$\_s*\\\"\z(.\{-1,\}\)\\\"" matchgroup=shRedir end="^\z1\s*$"
+syn region shHereDoc matchgroup=shRedir start="<<-\s*\\\\\_$\_s*\\\"\z(.\{-1,\}\)\\\"" matchgroup=shRedir end="^\s*\z1\s*$"
+
+syn match shsPrereq contained "\<[A-Z_,]\+\>" nextgroup=shsTestTitle
+syn match shsPrereqLazy contained "\<[A-Z_,]\+\>" nextgroup=shsTestBody
+
+syn cluster shCommandSubList add=shsTest,shsStatement
+
+hi def link shsStatement Statement
+hi def link shsTestStatement Function
+hi def link shsPrereq Identifier
+hi def link shsPrereqLazy shsPrereq
+hi def link shsTestBody shRedir
+
+let b:current_syntax='sharness'