Monaco improvements (#15333)

- Create theme at runtime which follows the CSS variables of the site
- Disable a few opinionated Monaco defaults like minimap and word highlights
- Move styles to separate file

Co-authored-by: zeripath <art27@cantab.net>

web_src/less/index.less

@@ -6,6 +6,7 @@
 @import "./features/animations.less";
 @import "./features/heatmap.less";
 @import "./features/imagediff.less";
+@import "./features/codeeditor.less";
 @import "./markdown/mermaid.less";
 
 @import "./chroma/base.less";