PostCSS/esbuild/Vite token-fallback plugins incorrectly rewrite var()-like text inside strings

[simison]

The design-token fallback plugins are inconsistent in how they handle var(--wpds-*) references that appear inside string literals (e.g. content: "var(--wpds-border-radius-sm)";) rather than as actual CSS var() functions.

Discovered while reviewing #80401 (comment thread)

Lightning CSS's behavior is already correct:

Plugin	Behavior
PostCSS	Regexp-based — matches and rewrites the text anywhere in the declaration, including inside quoted strings.
esbuild / Vite	Skip CSS files but rewrite the same text anywhere in JS/TS source.
Lightning CSS	AST-based — only adds fallbacks to parsed var() functions; leaves plain strings untouched.

Ask: Align the PostCSS (and esbuild/Vite) plugins in packages/theme/postcss-plugins (and equivalents) to only inject fallbacks for actual var() function references, not incidental text matches inside strings/comments, matching Lightning CSS's AST-aware behavior.

[mirka]

I'll note that this was a conscious trade off when implementing. I remember it being rather clear at the time that it wasn't worth the trouble. There should be a conscious decision again here, if it's really worth the extra effort and bug surface.