Extract CodeMirror styles (#17960)

- Extract CodeMirror-related styles to separate files
- Generalize CodeMirror styles where possible
- Improve fullscreen and side-by-side mode for dark theme

Co-authored-by: 6543 <6543@obermui.de>

web_src/less/index.less

@@ -16,6 +16,8 @@
 
 @import "./chroma/base.less";
 @import "./chroma/light.less";
+@import "./codemirror/base.less";
+@import "./codemirror/light.less";
 
 @import "_svg";
 @import "_tribute";