Treat `.pcss` files as CSS (#21402)

This addresses
https://github.com/zed-industries/zed/pull/19416#discussion_r1865019293
and also follows the [associated PostCSS file extensions for VS
Code](https://github.com/csstools/postcss-language/blob/5d003170c5ed962b09b9a0f3725a6cae885df292/package.json#L37).

Release Notes:

- `.pcss` files are now recognized as CSS

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Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>