Differential Revision: https://phabricator.services.mozilla.com/D142159

MozillaBuild 4.0 changed its `start-shell.bat` API for consumers that
want to run a command within it, while keeping their current terminal
active.

For a little bit, compatibility with `MACH_PS1_USE_MOZILLABUILD` will be
maintained, though the recent work in bug 1740123 should entirely
obsolete it. It's staying around //for now// so that regressions in
1740123 don't unnecessarily impede workflows.

However, `MACH_PS1_USE_MOZILLABUILD` has some serious drawbacks already
(can't have whitespace in arguments, can't pass braces to Mach, etc).
This patch still has these drawbacks, because they're tricky to solve.

In a couple months, once bug 1740123 has settled nicely, we'll remove
support for `MACH_PS1_USE_MOZILLABUILD`.

Differential Revision: https://phabricator.services.mozilla.com/D136544

When installing from Python.org, the `python` command isn't available by
default (in favour of `py`). Support using `py` instead.

There were issues running `hg` when `python` isn't added to the `PATH`,
so the docs are being updated accordingly.
Of course, it's still worth supporting `py`-instead-of-`python`
workflows for `git` users who won't be impacted by that bug.

Differential Revision: https://phabricator.services.mozilla.com/D136286

Assuming that the `MOZILLABUILD` environment variable is set, allow
invoking Mach from non-MozillaBuild terminals.

Note that MozillaBuild still needs to be installed, and the
`MOZILLABUILD` environment variable will have to be set.

For future reference: when I tried setting this up with Windows
Store's Python 3.9, I encountered issues when running binaries installed
via `pip`: it would fail with `abort: failed to load Python DLL
python3x.dll`.

Differential Revision: https://phabricator.services.mozilla.com/D133936

Bug 1686256 - Add Windows friendly wrapper for mach r=mhentges,firefox-build-system-reviewers,dmajor

This allows running mach on PowerShell for better Windows build experience. It currently has a limitation that blocks any whitespace use but such use should be rare enough.

Differential Revision: https://phabricator.services.mozilla.com/D101673