DEWP: Check for magic comments before minification (#65582)

Look for magic wp:polyfill comments before minification.

Avoids having to configure Terser to preserve these comments, and also avoids having them in the minified JS.

Minifiers run in the processAssets hook at the
`PROCESS_ASSETS_STAGE_OPTIMIZE_SIZE` stage. If we hook at the `PROCESS_ASSETS_STAGE_OPTIMIZE_COMPATIBILITY` stage to check for the magic comments, we don't have to worry about configuring Terser to preserve them (and won't have to have them making the output slightly larger either).

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Co-authored-by: anomiex <bjorsch@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>
Co-authored-by: sirreal <jonsurrell@git.wordpress.org>
Co-authored-by: sgomes <sergiomdgomes@git.wordpress.org>