CSS: var() fallbacks that contain commas are truncated to invalid CSS

### Bug description

`asCssValueWithDefault` in `src/vs/base/browser/cssValue.ts` truncates a `var(...)` fallback at its second comma:

```
asCssValueWithDefault('var(--x, rgba(0, 0, 0, 0.5))', 'white')
```

produces `var(--x, rgba(0)` - an unbalanced fragment that the CSS parser silently drops, so the whole declaration is lost instead of falling back.

Commas inside functions are part of the fallback per the css-variables spec; only the first top level comma separates variable name from fallback.

### Steps to reproduce

1. Call `asCssValueWithDefault('var(--x, rgba(0, 0, 0, 0.5))', 'white')`.
2. Observe the returned string is truncated at the second comma of the rgba() call.

### Expected behavior

The full fallback is preserved: `var(--x, rgba(0, 0, 0, 0.5))`.

### Version tested

Reproduced against compiled output on `main` (commit `38ec3d57f91b`). A fix is ready that splits at the first comma only and keeps everything after it as the fallback, with regression tests.