Checkin Policies only work 32bit mode and for checkin and rcheckin an
appropriate note on how to fix the problem is shown to the user if there
are problems detected when evaluating the checkin policies.

Show the same note in `git-tfs shelve` when the user explicitely
requested via the commandline parameter `-p` to evaluate the checkin
policies.

Visual Studio versions >= 2017 (or is it actually 2015 ?) store the
necessary information for e.g. CheckinPolicies no longer in the registry.
Instead, they use a private registry usually found in

  C:\Users\<USER>\AppData\Local\Microsoft\VisualStudio\15.0_xxxxxx\privateregistry.bin

where the xxxxxx value is differing between the different installed Visual
Studio editions and/or minor versions.

This commit introduces the ExternalSettingsManager which loads this private registry
and we can then add the VS extension folders gathered from the SettingsStore to
our assembly search path.

Sadly, this only works for 32bit process and crashes in a 64bit process as soon
as a method tries to create an ExternalSettingsManager instance. Therefore
this is put behind a check so we only do it in a 32bit process.

This implies that the checkin policies are only working in 32bit mode!

In addition to the search patsh gathered from the ExternalSettingsManager, add
  <vsInstallDir>\Common7\IDE\CommonExtensions\Microsoft\TeamFoundation\Team Explorer
to the search path, as otherwise some of the checkin policies used in my dev
environment here didn't load because a library in that folder wasn't
found. This indicated to me that we still are not 100% compatibile to
what Visual Studio does, but at least this fixes all the custom checkin
policies here, so it can't be that bad.

Add seperate sections on checkin policies for VS2017 and VS2019 and
explain how loading a registry hive works so it might get easier for someone
to futher debug into failures for the checkin policies.

Furthermore, add a section that checkin policies for VS2015 are not
supported.

The old content is left intact, as it contains information which might
still be valuable, but it is marked as obsolete so it is clear that
it is only for older git-fs versions.