Add project detection via search paths and pyproject.toml support

[pimdh]

Summary

This PR adds three key improvements to Pixi project discovery:

1. Configurable search paths: Discover Pixi environments using python-env.globalSearchPaths and python-env.workspaceSearchPaths, and legacy python.venvPath / venvFolders.
2. Directory traversal: Depth-limited scan (two levels) with heavy-directory skips and deduplication, shared between discovery sites
3. pyproject.toml support: Detect Pixi projects with a [tool.pixi] section in pyproject.toml in addition to pixi.toml

Changes

Search paths support

• Duplicated getAllExtraSearchPaths() from https://github.com/microsoft/vscode-python-environments/blob/main/src/managers/common/nativePythonFinder.ts (see Expose helper for resolving python-env search paths (getAllExtraSearchPaths) microsoft/vscode-python-environments#960 ) to get search paths from workspaces, python-env.globalSearchPaths, python-env.workspaceSearchPaths, and legacy python.venvPath / venvFolders.
• Added shared traversal helper that walks up to two levels deep, skips heavy directories (e.g. .git, node_modules), and deduplicates results
• Updated PixiEnvManager to reuse the shared traversal helper, avoid duplicate scanning of open workspace folders, and emit removals when paths disappear
• Automatically expands ~ in paths and resolves relative paths
• Validates paths before searching to avoid errors

pyproject.toml detection

• Updated refreshPixi() to detect projects using pyproject.toml with [tool.pixi] section
• Checks for both pixi.toml and pyproject.toml with [tool.pixi] when validating project directories
• Returns early if neither file is found, avoiding unnecessary pixi info calls

Fixes #23 and fixes #27

[pimdh]

Also fixes #27