Add .gitattributes to repository (crystal-lang#13479)

.gitattributes

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+## Sources
+
+*.cr text eol=lf
+
+## Sourced-in dependencies
+
+lib/** linguist-vendored
+
+## Generated files
+
+# produced by scripts/generate_windows_zone_names.cr
+src/crystal/system/win32/zone_names.cr linguist-generated
+# produced by scripts/generate_ssl_server_defaults.cr
+src/openssl/ssl/defaults.cr linguist-generated
+# produced by scripts/generate_grapheme_properties.cr
+src/string/grapheme/properties.cr linguist-generated
+# produced by scripts/generate_unicode_data.cr
+src/unicode/data.cr linguist-generated
+# produced by spec/generate_interpreter_spec.sh
+spec/interpreter_std_spec.cr linguist-generated
+# produced by scripts/generate_grapheme_break_specs.cr
+spec/std/string/grapheme_break_spec.cr linguist-generated
+# produced by spec/generate_wasm32_spec.sh
+spec/wasm32_std_spec.cr linguist-generated
+
+## Syntax highlighting
+
+Makefile.win linguist-language=makefile