Simplify CSSDataTypeParser::consumeFunctionBlock gradient parsing

Summary:
`CSSDataTypeParser<CSSRadialGradientFunction>::consumeFunctionBlock` (CCN 100) and `parseLinearGradientDirection` (CCN 24) parse CSS gradient syntax through deeply nested conditionals. This is a pure, behavior-preserving refactor: the recurring parsing steps (direction/position keyword resolution, shape-and-size, position-value lists) are extracted into focused helpers in `namespace facebook::react::detail`, matching the existing convention in this file, turning the two entry points into flat sequences. Parsing results are unchanged.

Changelog: [Internal]

Differential Revision: D108027809

Author: sammy-SC