fix(postcss_configs): support for resolve `postcss.config.json` file (#6651)
fix #6650
### Description
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Add support for `postcss.config.json` files so that they can be parsed
correctly.
### Testing Instructions
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Co-authored-by: Leah <8845940+ForsakenHarmony@users.noreply.github.com>
Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>