Add treesitter option

[vigoux]

This option can be used to enable treesitter features for a given buffer / globally.

For now the bare minimum is done, and is still very wip.

The roadmap is the following :

• [X] Create option
• [X] Setup highlighting when set treesitter=hl is done
• [ ] Remove highlighting when set treesitter= is done

[clason]

Is this something we want to revisit? Or is it superseded by nvim-treesitter's module system (i.e., this is the approach that should eventually come into core, when stable)?

[vigoux]

I personally prefer this approach, because it might be more useful when customizing how the tree-sitter C interface is used.

[clason]

We should revisit this as part of the "Lua highlighting in core" goal for 0.8.

(scope should be buffer-local, though?)

[lewis6991]

We should revisit this as part of the "Lua highlighting in core" goal for 0.8.

(scope should be buffer-local, though?)

It can only be buffer-local since extmarks are buffer local.

[clason]

An alternative would be to mirror the vim.lsp.start({...}) pattern, where the table could contain options like whether regex syntax is still run, the location of the parser and the queries, etc. (with reasonable defaults) that people can put in a manual FileType autocommand for 0.8.

(Later, we can set these up automatically.)