Support formatting cjs, cts, mjs, and mts files

Editor assessment

A model of economy: feat(fmt) with a subject that names exactly which extensions gain support (cjs, cts, mjs, mts), staying within limits and avoiding a trailing period. The scope is meaningful and the type will produce a correct minor-bump inference. My only stylistic note is that 'support formatting...' is a noun phrase; spec examples favor imperative verbs, so 'add formatting support for cjs, cts, mjs, and mts files' is marginally more canonical. The absent body is fine for a change of this size, though one sentence noting user-visible impact (e.g. `deno fmt` now touches these files) would help release notes. Overall a very clean header.