Tree-Sitter grammar for `.weir` files

**What problem does this solve?**

Large `.weir` files can get pretty dense. Syntax highlighting would help.

**Component**
- [ ] Core engine
- [ ] Plugin/Extension
- [x] Other: _____

It would actually be a new component. Possibly even an external component in its own repo.

Unlike a [TextMate](https://github.com/Automattic/harper/issues/2510) syntax highlighter, a Tree-Sitter grammar, would also be usable for grammar checking `.weird` files in Harper.

No files or tests are identified. Start by investigating the proposed Tree-Sitter grammar and deciding whether the new component belongs in Harper or an external repository, then compare its scope with the linked TextMate highlighter issue. Done means large .weir files have syntax highlighting and the grammar can also support checking the referenced .weird files.

Written by the indexing model from the issue text.