feat(editor): add .css language support

[kerbilg]

Hi,
this PR contains code to add css file support for the editor. It is more comfortable when editing themes.
I also added a screenshot from my development environment.

Thanks

[github-actions]

Size Change: +8.15 kB (0%)

Total Size: 2.25 MB

Filename	Size	Change
./dist/assets/Codemirror.js	122 kB	+8.16 kB (+7%)	🔍
./dist/assets/index.js	328 kB	-4 B (0%)
./dist/assets/Viewer.js	943 kB	-1 B (0%)
./dist/sw.js	6.29 kB	-1 B (0%)

ℹ️ View Unchanged

Filename	Size
./dist/assets/Codemirror.css	20 B
./dist/assets/Heightmap.js	120 kB
./dist/assets/Heightmap.css	282 B
./dist/assets/index.css	10.8 kB
./dist/assets/vendor.js	533 kB
./dist/assets/vendor.css	64.5 kB
./dist/assets/Viewer.css	244 B
./dist/config.json	25 B
./dist/fonts/roboto-black.woff2	11 kB
./dist/fonts/roboto-bold.woff2	11 kB
./dist/fonts/roboto-light.woff2	11.2 kB
./dist/fonts/roboto-medium.woff2	11.1 kB
./dist/fonts/roboto-regular.woff2	11 kB
./dist/fonts/roboto-thin.woff2	11 kB
./dist/img/icons/apple-touch-icon-180x180.png	6.47 kB
./dist/img/icons/favicon-16x16.png	355 B
./dist/img/icons/favicon-32x32.png	641 B
./dist/img/icons/icon-192-png	7.86 kB
./dist/img/icons/icon-512-png	33 kB
./dist/img/icons/mstile-150x150.png	2.24 kB
./dist/img/icons/safari-pinned-tab.svg	1.38 kB
./dist/img/klipper.svg	2.38 kB
./dist/img/logo.svg	295 B
./dist/img/sidebar-background.svg	929 B
./dist/index.html	715 B

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