Wait for extension contributions in welcome.showAllWalkthroughs

[yume-chan]

This PR fixes #135487

Another option is to hide Get Started: Open Walkthrough... and Get Started: Go Back commands from command palette (especially the go back, doesn't seem to be useful, as closeFindWidget command is not there)

[JacksonKearl]

Thanks for the PR! I agree Get Started: Go Back should not be in F1 menu, but I disagree that Get Started: Open Walkthrough... should not.

For this patch, if I remember correctly the issue with waiting for the extensions to be registered is that it may depend on the file system being scanned to facilitate the featuredFor flag, which could amount to a large delay. The real fix would be to provide the list without waiting for that scan, as we perhaps don't need it for this case. Or, configuring the quick pick to stream in walkthroughs as they are added.

[yume-chan]

Now I show the quick pick immediately with built-in walk throughs and a loading indicator, then update the quick pick items after all extension contributions are fetched.

[yume-chan]

The failed tests are from integrated terminal.

[JacksonKearl]

Thanks for working on this!