Make it easy to switch current md doc between preview and edit mode

[mjbvz]

Fixes #312425
Fixes #301686

Also adds a toggle preview command

[Copilot]

Pull request overview

This PR adds commands and UI contributions to quickly switch a Markdown document between the standard text editor and the Markdown preview custom editor, including a new toggle command and associated keybinding.

Changes:

• Set the custom editor preview tab icon for vscode.markdown.preview.editor.
• Add markdown.reopenAsPreview, markdown.reopenAsSource, and markdown.togglePreview commands and register them.
• Contribute new commands, editor title actions, and remap Shift+Ctrl/Cmd+V to the new toggle.

Show a summary per file

File	Description
extensions/markdown-language-features/src/preview/preview.ts	Sets the webview icon for the Markdown preview custom editor.
extensions/markdown-language-features/src/commands/reopenAsPreview.ts	Adds commands to reopen as preview/source and a toggle command.
extensions/markdown-language-features/src/commands/index.ts	Registers the newly added commands.
extensions/markdown-language-features/package.nls.json	Adds localized titles for the new commands.
extensions/markdown-language-features/package.json	Adds command/menu contributions and updates the default preview keybinding to toggle.

Copilot's findings

Comments suppressed due to low confidence (1)

extensions/markdown-language-features/package.json:372

• The shift+ctrl/cmd+v keybinding was changed to use resourceLangId. In keybinding contexts, resourceLangId can be set from non-editor focus (e.g. Explorer selection), which can make this shortcut active outside the editor and run markdown.togglePreview against whatever editor is currently active. To keep the shortcut scoped to the active markdown editor/preview, consider using editorLangId =~ ... for the text editor case and activeCustomEditorId == 'vscode.markdown.preview.editor' (and/or activeWebviewPanelId == 'markdown.preview') for preview cases instead of resourceLangId.

        "command": "markdown.togglePreview",
        "key": "shift+ctrl+v",
        "mac": "shift+cmd+v",
        "when": "resourceLangId =~ /^(markdown|prompt|instructions|chatagent|skill)$/ && !notebookEditorFocused"
      }

• Files reviewed: 5/5 changed files
• Comments generated: 2

[paige-clark]

Awesome!! Thank you Matt!