story: Highlight the Editor story with tree-sitter on native (#2694)

## Summary

The Editor story swapped the built-in tree-sitter highlighter for a
`syntect` adapter so WASM builds, which ship without tree-sitter
grammars, still get highlighting. Native paid for it, and the story
became visibly laggy while typing and scrolling.

`syntect` has no incremental mode, so each keystroke:

- reparsed the whole document on the main thread,
- copied the rope into a `String` twice — once for parsing, once more
for the full-document brace scan, because `fold_ranges_for_edit` was not
implemented,

and every frame `styles()` scanned all document tokens to resolve the
visible ones.

This limits the `syntect` adapter to `target_family = "wasm"`. Native
goes back to the built-in tree-sitter highlighter, which parses
incrementally on a background thread behind a 150ms debounce and queries
only the visible range. `syntect` moved into the WASM target
dependencies, so native never builds it at all.

The adapter is also trimmed, since WASM still runs it (both the story
copy and the Base showcase copy):

- one shared `SyntaxSet` in a `LazyLock`, instead of deserializing the
default syntax definitions for every instance
- fold ranges computed during `update` and `fold_ranges_for_edit`
implemented, removing the extra full-document string copy and brace scan
per edit
- `resolve_styles` locates the first overlapping highlight by binary
search instead of scanning every token on every frame
- semantic names cached per `Scope`, instead of `build_string` for every
scope of every token

No public API changes.

## Test Plan

- `cargo check -p gpui-component-story`
- `cargo check --target wasm32-unknown-unknown` in `crates/story-web`
- `cargo check -p gpui-base --example base_components`
- `cargo +nightly check --target wasm32-unknown-unknown` in
`crates/base/examples/wasm`
- `cargo machete`
- `cargo run` and typing in the Editor story

## AI Assistance

Diagnosis and the changes in this PR were written with Claude Code.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>