VSCE: Implement function to validate current branch

Created by: abeatrix

Close https://github.com/sourcegraph/sourcegraph/issues/34591

Summary

This PR implements a function to check if the current branch exists on the sourcegraph instance

If the user is currently on a branch that does not exist remotely, the links-commands will open the repo using the "HEAD" (aka main) branch unless a default branch has been set

Expected Behavior

For local files, users can choose between open / copy the main branch and the current branch.

• If the current branch does not exist remotely, it will fall back to the main branch.
• If the default branch is set, it will replace the current branch option when clicked.

For remote files, only current branch is available:

TODO

• Loom video: https://www.loom.com/share/b5e326959696455986cf68088a53b28a
• Test with different remote URL

Test plan

Manually tested. See loom video

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.