Create `nemo-vscode` VS Code extension

[geoffjay]

Summary

Create a VS Code extension that provides zero-configuration Nemo XML authoring support: registers nemo-lsp as the language server, adds TextMate grammar for syntax highlighting, and provides a "Preview in Nemo" command.

Context

VS Code users should get a complete authoring experience without manual configuration. The extension:

1. Activates on .xml files containing a <nemo> root element
2. Launches the nemo-lsp binary as the language server
3. Provides TextMate-based syntax highlighting (XML base + ${} expression token scopes)
4. Adds a "Preview in Nemo" command that saves the current file and launches the Nemo app

Acceptance Criteria

• Extension directory created at extensions/nemo-vscode/
• package.json with activation events for .xml files containing <nemo>
• Language server client configuration pointing to nemo-lsp binary
• TextMate grammar (syntaxes/nemo.tmLanguage.json):
  • XML element and attribute syntax (base)
  • ${} expression token scopes for interpolation highlighting
  • bind-* attribute patterns
  • Template references
• vscode-nemo contributes:
  • Language ID: nemo
  • File extension association: .xml (conditional on <nemo> content)
• "Preview in Nemo" command:
  • Saves the active editor
  • Runs nemo --app-config <current-file> in an integrated terminal
• README with installation and usage instructions
• Extension published to VS Code Marketplace (or prep for publishing with vsce)
• Works with nemo-lsp from PATH or configured path

Relevant Files

• crates/nemo-lsp/ — the LSP server binary
• schema/nemo.xsd — attribute groups and element structure for grammar design

Stack Base

• Stack on: main
• Blocked by: Create nemo-lsp language server #62 (nemo-lsp language server) — the extension depends on the LSP binary for completions, diagnostics, hover, etc.