Introducing FSharp.Editor.Tests

[psfinaki]

closes #14314

A new clean project dedicated for unit testing FSharp.Editor:

• No Salsa
• No legacy dependencies
• No weird file linking
• No pesticides

This also brings a few extra changes on top of the mere moving of the files:

• Turned on Fantomas hence formatting
• Removed a bunch of unused code
• Set proper namespaces in all files
• Added MS copywrites in all files
• Created FSharp.Editor.sln for light-weight work with the editor and its tests

What's beyond the scope:

• Further cleanup of VisualFSharp.UnitTests (there is a lot of cleanup potential there)
• Changing NUnit to xUnit
• Consolidating type XTests vs module YTests in suites

All successfully runs locally in about 3 minutes:

[TIHan]

My goodness, this is great.

[vzarytovskii]

Please, add the new test project to build scripts, under the corresponding switches, and make sure they show up in CI

[psfinaki]

Please, add the new test project to build scripts, under the corresponding switches, and make sure they show up in CI

Yep, all done, tests are running in CI: