Unify CSSCompoundDataType with its variant

[NickGerleman]

Summary:
Previously, CSS compound data types used two separate types:
CSSCompoundDataType<T1, T2, ...> as an empty marker struct for template
parameter deduction, and CSSVariantWithTypes<CSSCompoundDataType<T1, T2, ...>>
which resolved to std::variant<T1, T2, ...> for actual storage. This resulted
in redundant type aliases like CSSTransformFunctionVariant.

This change makes CSSCompoundDataType<T1, T2, ...> a using alias for
std::variant<T1, T2, ...> directly, so one type serves both roles: template
parameter deduction for parsing AND storage.

• Change CSSCompoundDataType from a marker struct to a using alias for
  std::variant
• Replace CSSValidCompoundDataType concept with is_variant_of_data_types
  trait that pattern-matches std::variant where all types satisfy CSSDataType
• Update merge_data_types and merge_variant specializations to use
  std::variant directly
• Store AllowedTypesT directly in CSSList compound type specialization
  (since it IS now the variant)
• Remove redundant CSSTransformFunctionVariant, CSSFilterFunctionVariant,
  and CSSBackgroundImageVariant aliases
• Update all downstream references in conversions

Changelog: [Internal]

Differential Revision: D94347088

[meta-codesync]

@NickGerleman has exported this pull request. If you are a Meta employee, you can view the originating Diff in D94347088.

[react-native-bot]

This pull request was successfully merged by @NickGerleman in 36212c2

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[meta-codesync]

This pull request has been merged in 36212c2.