This PR uses the `enhanced-resolve` instead of
`createRequire(…).resolve` which improves the usability when running the
upgrade tool locally using Bun.
While testing, we also noticed that it is not possible to use a
`cjs`-only plugin inside of an `esm` project. It was also not possible
to use an `esm`-only plugin inside of a `cjs` project.
# Test plan
We added integration tests in both the CLI (the CLI is an mjs project)
and in the PostCSS (where we can configure a `cjs` and `esm` PostCSS
config) integration tests where we created an `esm` and `cjs` based
project with 4 plugins (`cjs`-only, `esm`-only, and TypeScript based
plugins: `cts`-only and `mts`-only).
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