feat: add Switch thumb-* and track-* color utilities

[patrickwehbe]

What

Adds two custom utilities to theme.css:

• thumb-* maps a color to the React Native Switch thumbColor prop
• track-* maps a color to the React Native Switch trackColor prop

Both follow the same @prop pattern as the existing tint-* utility, reading the value from --color-*.

Why

Fixes #1757. v4 provided thumb-* utilities and track color values for styling the Switch component, and these were not listed in the migration guide as intentionally removed. This restores that behavior in v5. The mapping approach (color to thumbColor / trackColor via @prop) matches what was suggested in the issue.

Tests

Added src/__tests__/switch.tsx covering thumb-black and track-black, following the existing test conventions. The full suite passes:

Test Suites: 8 passed, 8 total
Tests:       37 passed, 37 total

[danstepanov]

Preemptive review to help triage for the 5.0 release, @marklawlor makes the final call.

Right intent and right place, but this doesn't work as written. The @prop at rule was renamed @nativeMapping in react-native-css 3.0.0 and the compiler silently ignores @prop now, so the color lands in style.thumbColor which Switch ignores, and the tests assert exactly that broken output. Also trackColor on RN Switch is an object with false and true keys, not a flat color, so it needs the nested paths trackColor.true and trackColor.false, which @nativeMapping supports. Swap @prop for @nativeMapping, map the track keys, and update the tests to assert top level props like @prop.tsx does. Separate heads up, the existing tint utilities in theme.css use the same dead @prop line, so tint-* looks broken on main today too. Filed as #1837.