Add new VS Code theme for mermaid diagrams

[mjbvz]

Fixes #316968

Creates a new theme for mermaid diagrams that is derived from the current VS Code color theme

Also makes sure this works for diagrams opened in their own window and in chats

[Copilot]

Pull request overview

This PR adds a new Mermaid theme option ("vscode") that derives Mermaid themeVariables from the active VS Code color theme so diagrams update correctly when the user changes themes, including in standalone diagram editors and chat-rendered diagrams.

Changes:

• Introduces a VS Code-derived Mermaid theme ("vscode") and makes it the default for both light and dark modes.
• Injects Mermaid extension configuration into editor/chat webviews so webview-side rendering can resolve the correct theme.
• Adds a webview-side theme tracker that recomputes Mermaid themeVariables from VS Code CSS variables and triggers rerendering on theme changes.

Show a summary per file

File	Description
extensions/mermaid-markdown-features/src/markdownMermaid/config.ts	Switches default theme to vscode and factors config-span rendering into reusable helpers.
extensions/mermaid-markdown-features/src/editorManager.ts	Injects the Mermaid config span into the standalone diagram editor webview HTML.
extensions/mermaid-markdown-features/src/chatOutputRenderer.ts	Injects the Mermaid config span into the chat output webview HTML.
extensions/mermaid-markdown-features/preview-src/shared/vsCodeTheme.ts	Adds the VS Code-derived Mermaid theme variable computation and DOM mutation tracking.
extensions/mermaid-markdown-features/preview-src/shared/index.ts	Updates defaults to vscode and centralizes Mermaid config building via the theme tracker.
extensions/mermaid-markdown-features/preview-src/markdown/index.ts	Rebuilds Mermaid config using the theme tracker before rerendering on preview refresh events.
extensions/mermaid-markdown-features/preview-src/chat/mermaidWebview.ts	Re-renders chat/editor diagrams when VS Code theme CSS variables/classes change using the new tracker.
extensions/mermaid-markdown-features/package.nls.json	Adds localized descriptions for the new theme option and existing theme choices.
extensions/mermaid-markdown-features/package.json	Adds vscode theme to settings enums (with descriptions), sets it as default, and marks notebook renderer messaging as optional.

Copilot's findings

Comments suppressed due to low confidence (1)

extensions/mermaid-markdown-features/preview-src/shared/vsCodeTheme.ts:191

• --vscode-chart-line does not appear to be a valid VS Code theme CSS variable (the usual pattern is --vscode-charts-lines). Because it’s listed first in the fallback chain, it may resolve to an inherited/default color and block the intended --vscode-charts-lines fallback. Please remove/fix this token name in the lineColor/border fallbacks.

	// Canvas / text
	set('background', '--vscode-editor-background');
	set('textColor', '--vscode-charts-foreground', '--vscode-editor-foreground', '--vscode-foreground');
	set('lineColor', '--vscode-chart-line', '--vscode-charts-lines', '--vscode-editor-foreground', '--vscode-foreground');

	// Primary (default node) colors
	set('primaryColor', '--vscode-editorWidget-background');
	set('primaryTextColor', '--vscode-charts-foreground', '--vscode-editor-foreground', '--vscode-foreground');
	set('primaryBorderColor', '--vscode-chart-line', '--vscode-editorWidget-border', '--vscode-focusBorder');
	set('mainBkg', '--vscode-editorWidget-background');
	set('nodeBorder', '--vscode-chart-line', '--vscode-editorWidget-border', '--vscode-focusBorder');

• Files reviewed: 10/10 changed files
• Comments generated: 1