feat(grammar): support "trim" for script heredocs

[jamessan]

Allow the optional "trim", as documented in ":help :lua-heredoc".

This removes the need for try_lex_heredoc_marker()'s is_let_heredoc
parameter since both script and let heredocs can have a parameter.

[clason]

Worth it for the scanner simplifications alone. (Be advised that this parser is not in a great shape and in sore need of a massive rework -- especially the scanner -- before you put more work into it.)

Needs a rebase now -- and if this affects queries, please make the appropriate changes in the same PR so I can cherry-pick them for nvim-treesitter (and then Neovim).

[clason]

Meaning, should the trim be highlighted?

[jamessan]

I don't think it affects queries. There's a new child node for script if trim is used, but it doesn't change anything else in the tree.

[clason]

It's not breaking, but you presumably added it to give it syntax highlighting?

[jamessan]

I added it so the parsing didn't break and cause the rest of my vim file to go un-highlighted. :)

I'll look into having it highlighted the same way as the let-heredoc trim is.

[clason]

If in doubt, @keyword.operator (the exact capture is not as important as having coverage to catch breaking changes).

[jamessan]

I used @keyword to match the let highlighting. Would you rather they be changed to @keyword.operator?

[clason]

Hmm, good question. I don't even know what this does.

[clason]

Yeah, probably should be @keyword.operator. But it doesn't matter here; these are just reference queries where only the patterns are important.