fix(optimize-css): only remove bx-- prefixed classes

[metonym]

Fixes #55

Currently, the optimizeCss plugin is too aggressive. It will remove non-Carbon prefixed selectors.

The fix is to first check that selectors actually include the Carbon bx-- prefix. This is an even more conservative approach, However, the critical mandate is to avoid false positives.

With this change, I've tested that the following selectors are not removed:

• Scoped Svelte style
• Global Svelte style (e.g., :global())
• Global selector from CSS StyleSheet
• Global selector overriding a Carbon class