Support for beta releases

VSCode doesn't currently support beta releases, but according to https://github.com/microsoft/vscode/issues/15756 it might be coming soon. However, that fix requires us to play games with our release version numbers since the vscode extension marketplace doesn't support prerelease data in version numbers. I'd rather not have to do things like "even minor numbers mean primary release, odd minor numbers mean prerelease".

Here are the options right now as I see them:

1. Do nothing. If people want to try a prerelease version, they will just manually download and install the vsix from our releases page. The downside is, users must manually install every upgraded version after this.

2. A hybrid of option 1 where we add code to the prerelease extension that checks for new versions and auto-updates behind the scenes. The downside is, that logic is already build into the vscode extension flows anyway, so we'd be re-inventing the wheel and running custom code.

3. Install a separate "BrightScript Language (Insiders)" channel to the extension store. The downside is that we'd have two store versions which might get confusing and clutters up the store.

4. Use the new insiders extension feature added in https://github.com/microsoft/vscode/issues/15756 and leverage the even/odd minor version pattern.

5. Wait 12-18 months for the extension marketplace to support prerelease semver patterns, and then implement https://github.com/microsoft/vscode/issues/15756.

   https://github.com/microsoft/vscode/issues/15756#issuecomment-916728348 (Long term 18 months (upstream dependency): MarketPlace will add support for pre-release versions)

The issue names no repository files or tests. Start by reviewing the linked VS Code prerelease discussion and the extension's existing release and Marketplace flow. Done would require choosing and implementing a supported prerelease distribution strategy, with versioning and upgrade behavior defined.