add grammar feature flag

[Rudxain]

I think it might be a good idea to make all tree-sitter grammar features opt-out. This has several benefits:

• Faster rebuilding without cache, for quickly testing hx "in isolation"
• Improved security and reliability. Because I've seen reports of segfaults. I feel uneasy running sudo hx knowing that there are grammars installed, so I typically use sudo nano instead.
• Better runtime perf in all kinds of resource usage (time, space, energy)

This gate should be enabled by default, to keep things as they are. If the flag is off, then hx is built like this:

• No :tree-sitter* commands
• No grammar-aware motions
• No -{g,-grammar} CLI flag, to prevent accidental fetching, and to avoid calling a C compiler with build
• xtask must not download or build any grammars
• etc...

I'd be willing to help a bit, by adding the relevant cfg attributes. I have a tight schedule this month, so I can't promise anything :(

Related: PR #8607 has been one of the cases where the lsp and dap feature-flags are useful

[the-mikedavis]

Feature flagging LSP and DAP makes some sense since those features don't make sense in a WASM build (#8607). It's possible to load tree-sitter parsers from WASM so this feature would be enabled for a WASM build.

Wrapping any and everywhere we use tree-sitter (motions, bracket matching, commands) with #[cfg(feature = "tree-sitter")] would be a much much larger change than you might think. And you can already effectively disable tree-sitter by removing $HELIX_RUNTIME/grammars/* and (chmod a-w that directory) since the lack of a parser and queries for a language is handled gracefully everywhere. I don't think we get much value out of feature-flagging and I forsee feature-flagging tree-sitter being a pain so I'm not interested in maintaining this.

[Rudxain]

Fair enough. Also, when #6633 is merged, the gate will be less useful, because filtering will be enough for most use-cases