Add runtime check to isLookbehindSupported() to prevent SyntaxError on older browsers (#8353)

The UA-based isLookbehindSupported() can return true for unrecognized
browsers (e.g. in-app WebViews, Chrome on old iOS using WebKit), causing
highlight.ts to load highlightjs-roc which contains regex literals with
lookbehind assertions. This triggers a fatal SyntaxError during module
parsing on browsers that don't support lookbehinds.

Adding areAllRegExpFeaturesSupported() as a runtime check before UA
sniffing ensures the guard works correctly even when UA detection fails.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: iHiD

app/javascript/utils/regex-check.ts

@@ -37,6 +37,8 @@ function compareVersions(versionA: string, versionB: string): number {
 }
 
 export function isLookbehindSupported(): boolean {
+  if (!areAllRegExpFeaturesSupported()) return false
+
   const parser = new UAParser().getBrowser()
 
   switch (parser.name) {