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>

options/locale/locale_en-US.json

@@ -213,6 +213,9 @@
   "editor.buttons.switch_to_legacy.tooltip": "Use the legacy editor instead",
   "editor.buttons.enable_monospace_font": "Enable monospace font",
   "editor.buttons.disable_monospace_font": "Disable monospace font",
+  "editor.code_editor.command_palette": "Command Palette",
+  "editor.code_editor.find": "Find",
+  "editor.code_editor.placeholder": "Enter file content here",
   "filter.string.asc": "A–Z",
   "filter.string.desc": "Z–A",
   "error.occurred": "An error occurred",
@@ -2250,8 +2253,6 @@
   "repo.settings.webhook.delivery.success": "An event has been added to the delivery queue. It may take few seconds before it shows up in the delivery history.",
   "repo.settings.githooks_desc": "Git Hooks are powered by Git itself. You can edit hook files below to set up custom operations.",
   "repo.settings.githook_edit_desc": "If the hook is inactive, sample content will be presented. Leaving content to an empty value will disable this hook.",
-  "repo.settings.githook_name": "Hook Name",
-  "repo.settings.githook_content": "Hook Content",
   "repo.settings.update_githook": "Update Hook",
   "repo.settings.add_webhook_desc": "Gitea will send <code>POST</code> requests with a specified content type to the target URL. Read more in the <a target=\"_blank\" rel=\"noopener noreferrer\" href=\"%s\">webhooks guide</a>.",
   "repo.settings.payload_url": "Target URL",