feat: linear gradient transition hint syntax

[intergalacticspacehighway]

Summary:

• Adds support for color transition hint syntax in linear gradients. e.g. linear-gradient(red, 20%, green).
• Simplified color stops parsing.
• The processColorTransitionHint is put into native code so it can support platform colors.
• Follows CSS spec (Refer transition hint section) and implementation is referred from blink engine source.

Changelog:

[GENERAL] [ADDED] - Linear gradient color transition hint syntax support.

Test Plan:

Added testcase in processBackgroundImage-test.ts and example in LinearGradientExample.js

Thoughts

Linear gradient fully supports CSS syntax with percentage value color stop positions and this PR adds transition hint support so can we remove experimental status from it?

px support can be done but we'll need to move color stop fixup algorithm from JS to native as we'll need view dimensions to support mixing percentage and px values. I guess we can hold that until new CSS parser?

[intergalacticspacehighway]

Closed in favor of #48410

This one has some bad merge history and some merge restrictions are getting triggered.