Name already in use

…arkdown

Move test plan into markdown file

Add new repositories to a highlighted user defined list

…ials

Simplify the credentials class, and clear up impossible error cases

And fix a typo

Sort Gist files by user-defined sort order

This moves our existing test plan under a "Required testing" section.

We're also adding the scenarios used for testing live results under an "Optional testing" section.

It's now only used from one place and inlining it doesn't make
getOctokit too long to be unclear.

I believe this doesn't change the user-visible behaviour at all. The user
won't be prompted to log in any more or less often than they would have
done before.

One benefit of this is that we can remove the registerListeners method
because we no longer need to know if the cached octokit is still valid.
Instead we just call vscode.authentication.getSession every time and it
will return the current session, which might be different from the last
time we called it. This might prompt the user to log in, but that would
have happened anyway because when the session changed we would have
overwritten our cached octokit instance.

Another benefit is that we no longer need the extension context and this
removed a surprisingly large amount of code where we are passing this
parameter around because we need it for the credentials.

The only downside I can see is that we call getSession more often and
create more javascript objects in general. I believe the performance
impact of this will be negligible and not worth worrying about.