Allow Lists of Metadata

[InsertCreativityHere]

This PR fixes #3459. We now support listing multiple metadata in these ways:

["foo", "bar"]
["foo"] ["bar"]
["foo", "bar"] ["baz"] // even this!

I kept the escaping/metadata tests as-is (ie. they're still using the comma syntax),
but updated everywhere else to use this 'new' syntax.
In my opinion, this 'new' syntax should be preferred going forward (as it's the syntax we use in IceRPC).

[InsertCreativityHere]

Note for reviewers:
Grammar.cpp is generated code. Don't waste your time reviewing it.

[bernardnormier]

I assume you kept some instances (tests) for the "," syntax?

[InsertCreativityHere]

Yes, the escape tests all still use the old syntax, and so do the metadata-specific tests in cpp/test/Slice.
Everything else was updated.