feat(css): emit named exports for JS keyword class names in CSS modules

[semimikoh]

CSS modules containing class names that match JavaScript reserved keywords
(e.g. switch, if, for, class) were not emitted as named exports,
making them accessible only via the default export object.

Summary

• dataToEsm skips named export generation when a key fails the
  key === makeLegalIdentifier(key) check — reserved words like switch
  get prefixed to _switch, so they never match
• Added includeArbitraryNames: true option to the dataToEsm call in
  cssPostPlugin, which emits string-named exports for such keys:
  export { _arbitrary0 as "switch" }
• Reserved keyword class names are now accessible via namespace import
  (styles.switch) or string literal import (import { "switch" as s })

Related

Fixes #14050

Test plan

Added playground test case with a CSS module containing a .switch class,
verifying it is correctly applied via import * as keywordMod and
keywordMod.switch

[sapphi-red]

I think this will break existing apps when the app targets browsers that does not support the arbitrary names feature.
I guess we need to conditionally enable this when the target supports them. or maybe we can transpile it somehow.