Expose TLA+ language model tools via Model Context Protocol server

Add support for the Model Context Protocol (MCP) which enables TLA+ language
model tools to be used by editors that implement MCP but don't support VS Code's
native LanguageModelTool API - specifically Cursor editor.

The implementation:
- Adds an MCP server exposing four TLA+ tools (parse, symbol, check, smoke)
- Makes the server configurable via a new `tlaplus.mcp.port` setting
- Ensures the server starts/stops properly with the extension lifecycle
- Integrates @modelcontextprotocol/sdk dependency for MCP specification

References:
https://forum.cursor.com/t/support-languagemodeltool-api/91137
microsoft/vscode-discussions#2411 (comment)

[Feature]

Author: lemmy