feat(@angular/build): support custom postcss config

[BBboy01]

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

allow user define custom postcss configure from legacy json configure file type postcss.config.json and .postcssrc.json

Issue Number: N/A

What is the new behavior?

use modern js type postcss configure file type

Does this PR introduce a breaking change?

• Yes
• No

json type configure file can never be used, but this part is not include in doc, so may not is breaking changes.

Other information

Close #27819

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[BBboy01]

json file type has no reference of postcss plugins, and no lsp supports, js is prettey better way to use as configure file

and the early migration guide still recommand to use js

if we realy want to contain every file type of postcss configure, i prefer to use postcss-load-config

[alan-agius4]

Closing as CLA has not been signed.

[BBboy01]

@alan-agius4 sorry about the CLA, I'm not noticed the detail of robot comment, now the CLA check is passed