SCM: Bring back SCM Providers view

[joaomoreno]

From #102118

Let's bring back the SCM Providers view:

• It should look and feel the same as before
• Its selection should reflect the visibility of the repositories in the SCM View, instead of messing around with view panes

This will make it easier to manage many repositories.

[joaomoreno]

Pushed: 4cd3ec3

Good news

• The Source Control Repositories view is now back (it used to be called Source Control Providers. It lets you control visibility of multiple repos in the Source Control view.
• The setting scm.repositories.visible is now back (it used to be called scm.providers.visible).
• I've sprinkled visibility actions both in the context menu of the Source Control view as well as the ... menu, to make it easier to control the visibility without using the Source Control Repositories view.
• This will all be available in the next Insiders, hopefully this week.

Bad news

I'm out on vacation until the end of the month so no more work will happen this month here apart from critical bugfixing. Here's the state for now:

• The workbench opens the exact same way as today. On any given workspace, one must make the Source Control Repositories view visible manually. Once visible in a workspace, it will remain visible. I expect this to change and make the view visible depending on (1) the stored visibility state or (2) the number of repositories.
• No repository visibility state is stored yet. On reload, they will all be loaded once again. I expect this to change to simply remember which repositories were visible since the last session, per workspace.

[bpasero]

Very cool, I think this solution is elegant, will selfhost on it and report back if I find issues.