fix: bundle CSS from external dependencies instead of externalizing

[compass-vin]

Summary

Fixes #959

CSS files imported via JS/TS from production dependencies (listed in dependencies, peerDependencies, or optionalDependencies) were being marked as external by the deps plugin, which prevented the CSS plugin from processing them into the output bundle.

This affected both regular CSS imports and ?inline CSS imports:

• import 'jodit/es2021/jodit.css' — CSS not included in output style.css
• import style from 'jodit/es2021/jodit.css?inline' — CSS not inlined as string

Root cause: externalStrategy() in src/features/deps.ts checked if an import's package name matched a production dependency and externalized it unconditionally. CSS files need to be bundled so the CSS plugin can process them.

Fix: Added a guard in externalStrategy() that returns 'no-external' for CSS file imports (.css, .scss, .sass, .less, .styl, .stylus), stripping any query string (e.g. ?inline) before testing. This runs after the alwaysBundle check but before the dependency externalization checks.

Test plan

• Added test: external dependency CSS is bundled via JS import
• Added test: external dependency CSS is bundled via JS ?inline import
• All existing CSS tests pass (100 passed + 3 expected fail)
• All existing deps tests pass (15 passed)

🤖 Generated with Claude Code

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commit: 59da85c

[compass-vin]

Tested fix is here:

https://stackblitz.com/edit/github-2dddpu9g-22k8rjck?file=src%2Findex.ts

[compass-vin]

@sxzz Hey, it's my first time contributing to this project so apologies if I'm missing a step. Is there anything I need to do to get this reviewed?

Thanks for all the work on this btw, I love this library.