Turbopack: Add postcss.config.ts support (#89049)
### What?
Adds support for TypeScript PostCSS configuration files in Turbopack.
### Why?
Users can write their PostCSS configuration in TypeScript for better
type safety and IDE support. This was already supported in webpack but
was missing in Turbopack.
### How?
Added the following TypeScript config file names to Turbopack's PostCSS
config detection:
- `postcss.config.ts`, `postcss.config.mts`, `postcss.config.cts`
- `.postcssrc.ts`, `.postcssrc.mts`, `.postcssrc.cts`
- `.config/postcssrc.ts`, `.config/postcssrc.mts`,
`.config/postcssrc.cts`
Also updated the Turbopack documentation to reflect the supported config
file extensions (`.js`, `.mjs`, `.cjs`, `.ts`, `.mts`, `.cts`).
### Testing
Added e2e tests that verify the TypeScript PostCSS config is actually
loaded and applied by using a custom plugin that transforms colors.
Tests cover:
- `postcss.config.ts`
- `postcss.config.mts`
- `postcss.config.cts`
- `.postcssrc.ts`
