perf: load codemirror into a chunk for faster LCP

[pataar]

This loads codemirror into a seperate chuk for faster LCP.

More information: https://web.dev/lcp/

You can use the analyze label to see the difference in bundle size.

[github-actions]

Size Change: -5.74 kB (0%)

Total Size: 1.52 MB

Filename	Size	Change
./dist/assets/Heightmap.js	172 kB	-2 B (0%)
./dist/assets/index.js	245 kB	-1.68 kB (-1%)
./dist/assets/vendor.js	585 kB	-117 kB (-17%)	👏
./dist/assets/Viewer.js	282 kB	-2 B (0%)
./dist/index.html	637 B	-3 B (0%)
./dist/assets/Codemirror.js	113 kB	+113 kB (new file)	🆕
./dist/assets/Codemirror.css	20 B	+20 B (new file)	🆕

ℹ️ View Unchanged

Filename	Size
./dist/assets/Heightmap.css	74 B
./dist/assets/index.css	56.3 kB
./dist/assets/vendor.css	64.5 kB
./dist/assets/Viewer.css	318 B
./dist/config.json	25 B

_{compressed-size-action}