Support downloading "dev" for a certain release

[roblourens]

I'd like to be able to say vscode-dts dev 1.36 to make sure I get a proposed .dts which is compatible with my vscode.d.ts. The one from master is not guaranteed to be compatible with the vscode.d.ts from a release. This is probably mainly just useful for people like me who are actually shipping an extension that uses proposed, not just using those APIs for development.

[octref]

In next version you can use npx vscode-dts dev <git-tag | git-branch>.