Add C language support

[DreierF]

TanStack Highlight should ship an isolated language definition for C, with the fence names c and h.

We render documentation that contains C code blocks, and these currently fall back to plaintext. Registering c as an alias of cpp would be the cheapest option, but it is inaccurate: C code can use new, class, this, template, and namespace as ordinary identifiers, and the C++ definition highlights them as keywords. C also has keywords that C++ lacks, such as restrict, _Generic, _Static_assert, and _Atomic.

I have an implementation ready that follows the shape of the C++ module:

• src/languages/c.ts on the existing patternTokenizer, with no cross-language imports
• prefixed strings, character literals, include paths, preprocessor directives, C11 and C23 keywords and types, digit separators, and wb suffixes
• C++-only words stay plain
• tests, a size profile, a benchmark profile, a fixture, docs, and a changeset

Size: Core + C is 2.84 KB gzip. The all-language entry grows from 17.68 KB to 17.88 KB gzip. Core and the existing selective profiles are unchanged. pnpm run verify passes.

If you prefer the alias on cpp instead, I can reduce the PR to that.