feat(vscode): Prompt to get an access token when authentication fails

Created by: steverice

If connecting to the sourcegraph instance returns a 401, give the user two actions to take. The first will take the user to sourcegraph to create a new token, using the token callback flow added in #35826 if available. The second will open the workbench settings to allow the user to paste in a token.

This is particularly useful when distributing a sourcegraph.url setting as part of the workspace/repository to point to a private instance. Since the URL is already set, the user does not get the first-time setup view when activating the sourcegraph extension. Instead, they will get this error dialog:

Test plan

• Ran existing unit tests with yarn test
• Ran existing integration tests with yarn test-integration
• Walked through the flow of installing the vscode extension in a fresh instance with no settings, with a URL and token already set, with only a URL set, against both sourcegraph.com and a private instance

Created by: markjm

Review: Approved

Created by: slimsag

Review: Approved

LGTM, this looks great, thanks so much for sending this change!

Created by: jdorfman

Review: Approved

restored source branch github/fork/steverice/get-token