Remove need for internal VS APIs

[mrlacey]

Describe the bug

A few places (most notably SafeThreading.cs) make use of internal VS APIs. There are a few negative consequences of this:

• It makes testing harder
• It has lead to a poor abstraction between the extension code and VSEmulator
• It creates risk as it the code is dependent upon APIs which aren't officially supported and so could change at any time.
• Any change in these internal APIs can lead to the need to have the extension target specific versions of VS and so limit the usefulness of the extension when someone wanting to use it isn't able to update to the latest version of VS.

To Reproduce

See also #4491

Additional context

Applies to every extension as this is linked to core code logic :(

Applies to the following platforms:

• [X] WinUI
• [X] WPF
• [X] UWP

About your setup

• Visual Studio Version: 17.2
• Template Studio Wizard Version: 5.1
• Windows Build: 10 & 11