Refactor GitHub Actions workflows

[msujew]

Continues the work of #127

1. Removes all build & publishing scripts except for the one previously known as ovsx-solid---publish-vscode-built-in-extensions-specific-version. None of the deleted scripts were in use.
2. Runs build on PRs now.
3. Publishes the builtins via a workflow_dispatch trigger now (instead of pushing to a specific branch).

[tsmaeder]

The deleted workflows were mentioned in the documentation, right? So I assume they were meant to be triggered manually? Why do we not need them anymore?

[msujew]

@tsmaeder Some of them were supposed to be triggered on a nightly schedule, see also the comment in #127. Note that I'm not well versed in this repo - it just seems to me like aside from the single Action left in this PR, none of the other actions were in use.

In my mind, all of the deleted workflows cannot be aligned to the IP checks that we have to do. They check out arbitrary versions of VS Code and simply publish everything without performing IP checks. The only Action left is the one where we explicitly set the VS Code version, perform the IP checks with the Eclipse Foundation and then publish that specific version.

[marcdumais-work]

@tsmaeder Some of them were supposed to be triggered on a nightly schedule, see also the comment in #127.
In my mind, all of the deleted workflows cannot be aligned to the IP checks that we have to do. They check out arbitrary versions of VS Code and simply publish everything without performing IP checks.

That's correct - the two workflows that would build/publish nightly were in the relatively early days, before we performed regular IP Checks, and these workflows AFAIK have not been used in a few years... At the time, Theia could not reliably consume such bleeding edge built-ins anyway, lacking up-to-date vscode extensions API support.