Improve syntax highlighting

* Add ability to customize which edition is used for highlighting
  (`g:rust_edition`).
* Don't highlight enum variants that happen to have the same name as a
  prelude item. (Also applies to associated types in paths.)
* Add support for `safe fn`.
* Add support for `&raw const` and `&raw mut`.
* Add support for `gen` keyword.[^1]
* Highlight items added to the prelude in 2021 and 2024 editions.
* Update list of derive macros.
* Highlight keywords in positions where an item name is expected
  (e.g., `fn priv()`).[^2]
* Allow non-ASCII characters in identifiers.
* Allow `?` as a macro repetition operator (`$()?`).[^3]
* Allow raw identifiers in paths and function calls.
* Highlight `as` as a keyword rather than an operator.[^4]
* Improve highlighting of `async`, `await`, `try`, and `dyn`.
* Improve highlighting of `impl ... for ...` using proper parsing.
* Improve detection of turbofish function calls (`f::<T>()`).
* Improve parsing of attributes (fixes cases like `#[[]]`).
* Fix parsing of `/* */*` (previously parsed as a comment containing an
  unclosed nested comment).
* Improve detection of which Markdown code blocks should be highlighted
  as Rust code in doc comments.
* Fix parsing of multiline attributes and macro repetition expressions
  (`$(x)*`) inside code blocks in doc comments.
* Fix parsing of non-Rust code blocks in block doc comments (`/** */`).
* Fix parsing of doc comments containing an unclosed code block.
* Highlight leading `# ` in doc comment code blocks.
* Add option not to highlight code in doc comments
  (`g:rust_highlight_doc_code`).[^5]
* Reduce stale highlighting when syntactic constructs span multiple
  lines by setting `syn sync linebreaks` to 1.
* Add a distinct syntax group for prelude structs instead of including
  them in `rustTrait`.
* Reduce false positives in highlighting of assert and panic macros.
* Remove obsolete syntax.

[^1]: Fixes rust-lang#521
[^2]: Fixes rust-lang#406
[^3]: Fixes rust-lang#498
[^4]: Fixes rust-lang#493
[^5]: Fixes rust-lang#407

Author: taylordotfish