ci-windows: introduce manual windows CI Note: most of the foundational work of this commit was done by Matt Harbison, but enough has changed that I don't feel comfortable sending this patch as his. In our quest to remove Python 2 from Mercurial, we need to get Windows CI going to check that Python 3 support is up to an acceptable standard. This is the first step: adding a manual step to run a full Windows CI, with a certain definition of "full" since some things are not installed yet, like SVN. Differential Revision: https://phab.mercurial-scm.org/D11025