Pipeline-ify release process

[rjmholt]

Super-issue for #1921.

Supercedes #1425.

Pipeline should have steps:

• Manual step: do changelog generation and ensure versions are correct
• Manual step(?): Optionally create and push release branches
• Build PSES
• Build vscode-PS with PSES_BRANCH set
• Run tests on VSIX and PSES.zip
  • Validate certs
  • Validate structure
  • Run all tests on artefact
• Manual step: validate VSIX and authorise release
• Publish GitHub release
• Publish VSIX to marketplace
• Publish PSES to NuGet gallery
• Update ADS
• Open PRs to increment versions in source of PSES and vscode-PS

[andyleejordan]

We did it!!!