feat!: normalize extraction content sources

[antfu]

rework #2503, thanks to @sibbng

close #2503

Breaking changes:

• extraContent option is renamed to content
• include option is moved to content.pipeline.include
• exclude option is moved to content.pipeline.exclude
• content.plain is removed to content.inline
• inline content option in PostCSS plugin is removed in favor of the normalized content option in uno.config.ts

Features:

• content.inline now support providing async functions
• set content.pipeline to false can disable pipeline extract completely, close feat(webpack): add scan-only mode to adapt webpack5 persistent cache  #2652
• improve docs

For more details, please refer to https://unocss.dev/guide/extracting#content-sources

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