feat: add inline commands for theme build and watch in the themes view

[dermatz]

This pull request introduces improvements to the MageForge extension's theme management commands and enhances error handling for a better user experience. The main changes add new inline commands for building and watching themes directly from the view, improve error messaging for common environment issues, and refactor how the themes tree view is registered.

Theme command enhancements:

• Added new inline commands mageforge.theme.buildInline and mageforge.theme.watchInline with corresponding entries in package.json and registered them in the extension activation logic, allowing users to build or watch themes directly from the themes view. [1] [2] [3]
• Updated the way the themes tree view is registered by switching from registerTreeDataProvider to createTreeView, enabling richer view features.

Error handling improvements:

• Enhanced error reporting in execMageforge to provide a fallback message if both stderr and the error message are missing.
• Improved theme loading error messages by stripping ANSI codes and mapping common environment errors (e.g., DDEV, Docker, Lando, missing CLI) to user-friendly messages, with truncation for long messages to keep the UI clean. [1] [2]