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>

templates/repo/editor/edit.tmpl

@@ -21,7 +21,7 @@
 								<div class="flex-text-block tw-justify-between tw-flex-wrap">
 									<div class="ui compact small menu small-menu-items repo-editor-menu tw-self-start">
 										<a class="active item" data-tab="write">{{svg "octicon-code"}} {{if .IsNewFile}}{{ctx.Locale.Tr "repo.editor.new_file"}}{{else}}{{ctx.Locale.Tr "repo.editor.edit_file"}}{{end}}</a>
-										<a class="item" data-tab="preview" data-preview-url="{{.Repository.Link}}/markup" data-preview-context-ref="{{.RepoLink}}/src/{{.RefTypeNameSubURL}}">{{svg "octicon-eye"}} {{ctx.Locale.Tr "preview"}}</a>
+										<a class="item{{if not .CodeEditorConfig.Previewable}} tw-hidden{{end}}" data-tab="preview" data-preview-url="{{.Repository.Link}}/markup" data-preview-context-ref="{{.RepoLink}}/src/{{.RefTypeNameSubURL}}">{{svg "octicon-eye"}} {{ctx.Locale.Tr "preview"}}</a>
 										{{if not .IsNewFile}}
 										<a class="item" data-tab="diff" hx-params="context,content" hx-vals='{"context":"{{.BranchLink}}"}' hx-include="#edit_area" hx-swap="innerHTML" hx-target=".tab[data-tab='diff']" hx-indicator=".tab[data-tab='diff']" hx-post="{{.RepoLink}}/_preview/{{.BranchName | PathEscapeSegments}}/{{.TreePath | PathEscapeSegments}}">{{svg "octicon-diff"}} {{ctx.Locale.Tr "repo.editor.preview_changes"}}</a>
 										{{end}}
@@ -32,8 +32,8 @@
 							<div class="ui bottom attached segment tw-p-0">
 								<div class="ui active tab tw-rounded-b" data-tab="write">
 									<textarea id="edit_area" name="content" class="tw-hidden" data-id="repo-{{.Repository.Name}}-{{.TreePath}}"
-										data-previewable-extensions="{{StringUtils.Join $.CodeEditorConfig.PreviewableExtensions ","}}"
-										data-line-wrap-extensions="{{StringUtils.Join $.CodeEditorConfig.LineWrapExtensions ","}}">{{.FileContent}}</textarea>
+										data-code-editor-config="{{JsonUtils.EncodeToString $.CodeEditorConfig}}"
+										placeholder="{{ctx.Locale.Tr "editor.code_editor.placeholder"}}">{{.FileContent}}</textarea>
 									<div class="editor-loading is-loading"></div>
 								</div>
 								<div class="ui tab tw-px-4 tw-py-3" data-tab="preview">