Replace Monaco with CodeMirror (#36764)

- Replace monaco-editor with CodeMirror 6
- Add `--color-syntax-*` CSS variables for all syntax token types,
shared by CodeMirror, Chroma and EasyMDE
- Consolidate chroma CSS into a single theme-independent file
(`modules/chroma.css`)
- Syntax colors in the code editor now match the code view and
light/dark themes
- Code editor is now 12px instead of 14px font size to match code view
and GitHub
- Use a global style for kbd elements
- When editing existing files, focus will be on codemirror instead of
filename input.
- Keyboard shortcuts are roughtly the same as VSCode
- Add a "Find" button, useful for mobile
- Add context menu similar to Monaco
- Add a command palette (Ctrl/Cmd+Shift+P or F1) or via button
- Add clickable URLs via Ctrl/Cmd+click
- Add e2e test for the code editor
- Remove `window.codeEditors` global
- The main missing Monaco features are hover types and semantic rename
but these were not fully working because monaco operated only on single
files and only for JS/TS/HTML/CSS/JSON.

| | Monaco (main) | CodeMirror (cm) | Delta |
|---|---|---|---|
| **Build time** | 7.8s | 5.3s | **-32%** |
| **JS output** | 25 MB | 14 MB | **-44%** |
| **CSS output** | 1.2 MB | 1012 KB | **-17%** |
| **Total (no maps)** | 23.3 MB | 12.1 MB | **-48%** |

Fixes: #36311
Fixes: #14776
Fixes: #12171

<img width="1333" height="555" alt="image"
src="https://github.com/user-attachments/assets/f0fe3a28-1ed9-4f22-bf25-2b161501d7ce"
/>

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/css/features/codeeditor.css

@@ -1,47 +0,0 @@
-.editor-loading {
-  padding: 1rem;
-  text-align: center;
-}
-
-.monaco-editor-container,
-.editor-loading.is-loading {
-  width: 100%;
-  min-height: 200px;
-  height: 90vh;
-}
-
-.edit.githook .monaco-editor-container {
-  border: 1px solid var(--color-secondary);
-  height: 70vh;
-}
-
-/* overwrite conflicting styles from fomantic */
-.monaco-editor-container .inputarea {
-  min-height: 0 !important;
-  margin: 0 !important;
-  padding: 0 !important;
-  resize: none !important;
-  border: none !important;
-  color: transparent !important;
-  background-color: transparent !important;
-}
-
-.monaco-editor,
-.monaco-editor .overflow-guard {
-  border-radius: 0 0 var(--border-radius) var(--border-radius);
-}
-
-/* fomantic styles destroy this element only visible on IOS, restore it */
-.monaco-editor .iPadShowKeyboard {
-  border: none !important;
-  width: 58px !important;
-  min-width: 0 !important;
-  height: 36px !important;
-  min-height: 0 !important;
-  margin: 0 !important;
-  padding: 0 !important;
-  position: absolute !important;
-  resize: none !important;
-  overflow: hidden !important;
-  border-radius: var(--border-radius-medium) !important;
-}