Add @vscode/python-environments npm package

[StellaHuang95]

Our team owns multiple VS Code extensions consuming the Python Environments API. Rather than each extension maintaining its own local copy of the type definitions, which leads to duplication, version drift, and silent breakage, this PR publishes the API as @vscode/python-environments on npm, matching the pattern already established by @vscode/python-extension.

The PR includes following changes:

1. The pythonEnvironmentsApi self-contained npm package contains:

src/main.ts: public API types + PythonEnvironments.api() helper
package.json, tsconfig.json, .npmignore, README.md, LICENSE.md, SECURITY.md,
basically the same structure as the VS Code Python Extension.

2. The CI pipeline azure-pipeline.npm.yml + test/pack templates, use microsoft/vscode-engineering shared npm-package template. Tests on Linux/macOS/Windows. Publish gated behind manual parameter (default: off).

Next steps:

• Register pipeline in Azure DevOps
• Dry-run pipeline (publishPythonEnvsApi: false) and verify everything is working as expected.

[StellaHuang95]

FYI, @edvilme