fix(typing): Fix clean-css types

[lanthaler]

See https://github.com/jakubpawlowicz/clean-css/blob/f813323d32e51087adda564a6745276269fc347c/docs/index.html#L106-L109 and https://github.com/jakubpawlowicz/clean-css/blob/22b2b2e59045a1072dc3e1be53ca0b55a7330875/test/integration-test.js#L512-L537 for the available values

Description

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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