v0.40.0

What's changed

Add a test GitHub tab to the My work flyout

Azure DevOps has no documented place for an extension in the top bar, so
there was no global way to see the GitHub pull requests you created or
were asked to review. The org's contribution tree shows that the My work
flyout (the checklist icon) is an ordinary ms.vss-web.tab-group,
ms.vss-tfs-web.my-work-pivot, with ADO's own Work items (900), Pull
requests (1000) and Favorites (1100) as ms.vss-web.tab children and no
restriction on who may contribute. This adds a "GitHub" tab at order
1050, mainly to find out whether the flyout renders an extension's
iframe tab at all (its own tabs are built-in components, not iframes)
and what context it hands it; the first load logs that to the console
as "[GitHub my work]".

The tab lists open pull requests across every repository the GitHub App
can see, in two cards like ADO's own: "Created by me" (author:@me) and
"Review requested" (review-requested:@me). It uses REST /search/issues
rather than GraphQL so it can be checked in cloud sessions too. Rows link
to github.com for now; opening them in our PR hub needs a map from
repository to the ADO project that connects it, which comes once the tab
is known to work. Signed-out users get a sign-in button.

Built from azure-devops-ui throughout (Card, Header, Table with
TwoLineTableCell, VssPersona, Pill, MessageCard, Button); nothing
hand-rolled. Mock harness: myWorkTab.html, with signedOut and rateLimit
scenarios.

_87b514a

Full changelog: v0.39.3...v0.40.0