vscode-v0.3.0

feat(vscode): publish to VS Marketplace + Open VSX

Adds .github/workflows/publish-vscode.yml (tag vscode-v*.. triggers a
build + publish to both the VS Marketplace and Open VSX), repository
metadata needed by vsce, and doc updates.

macOS note

The .dmg is not notarized, so macOS reports the app as "damaged".
After copying confy.app to Applications, clear the quarantine flag:

xattr -cr /Applications/confy.app

Full Changelog: v0.16.0...vscode-v0.3.0