feat(css): add cssVars prop and resolveCssVars middleware

[janpe]

Summary

Closes #2853. Supplies CSS custom properties from JS for SVGs that reference var(--name) without defining the variables inline — useful for theming or when prepending values to an SVG string received from elsewhere.

<SvgCss xml={svg} cssVars={{ '--brand': theme.primary }} />

How

Adds two new exports from react-native-svg/css:

• resolveCssVars(vars): Middleware — a new Middleware that resolves var() references on color-like props (color, fill, floodColor, lightingColor, stopColor, stroke) against a caller-supplied variable map. Reuses the existing cssSelect query and resolveElementVariables helpers from inlineStyles.
• CssVars — the variable map type.

The intended composition is to run inlineStyles first and resolveCssVars second, so <style>-declared variables win over caller-supplied ones:

parse(xml, (ast) => resolveCssVars(vars)(inlineStyles(ast)))

For convenience, SvgCss, SvgCssUri, SvgWithCss, and SvgWithCssUri accept an optional cssVars prop that performs the composition internally.

Scope

• Pure JS layer (src/css/css.tsx, src/css/index.tsx). No native code.
• inlineStyles is unchanged. The Middleware type is unchanged.
• Existing call sites (parse(xml, inlineStyles), <SvgCss xml={...} /> without cssVars) take exactly the same code path as before.

Test Plan

1. yarn jest css.test — three new tests pass:
   • resolveCssVars resolves var() when no <style> is present
   • composing resolveCssVars over inlineStyles lets <style> win
   • SvgCss renders with cssVars prop

Compatibility

OS	Implemented
iOS	✅
MacOS	✅
Android	✅
Web	✅

JS-only change — runs everywhere the existing SvgCss components run.

Checklist

• I have tested this on a device and a simulator
• I added documentation in USAGE.md (the project's user-facing docs; README.md links here)
• I updated the typed files (typescript)
• I added a test for the API in the __tests__ folder