[Discussion] Reintegrating fork features (nested classes, theory support, etc.)

Hello all. First off, thanks for the extension, has been very useful to me for some time now. This is not an issue per se, just wanted to start a discussion to gauge interest in merging some of the changes I implemented on a fork of this repository about 6 months ago.

Back then the repo seemed unmaintained so I forked it and implemented a few features that I wanted to have. Unfortunately, I also performed a relatively large refactoring so the current fork is not easily merged back to master. Hence, this issue.

The major features I added to the extension include:
1. Nested class comprehension and navigation (including proper tree grouping)
2. Proper theory comprehension (including tree grouping)
3. Showing test duration in code lens and the tree view (including a setting to turn it on/off)
4. Added ability to collect test coverage (visualization left to other extensions, such as [coverage-gutters](https://marketplace.visualstudio.com/items?itemName=ryanluker.vscode-coverage-gutters))
5. Added additional 'in-context' actions - 'Debug test in context' and 'Cover test in context', including support for keyboard shortcuts

I also split the test result file parsing into separate classes (per test framework) and added some tests for those. I also introduced stricter TSLint rules.

The complete fork in question is here if you want to take a look: https://github.com/mareklinka/vscode-dotnet-test-explorer
This version of the extension is not published anywhere, I just have it for my own personal use.

The main solution I was working on at the time was .NET Core 3.1 so I'm not 100% sure the changes I introduced would work with 2.2 and lower. But I wanted to bring this up in case any of the features would be interesting to the maintainers. If so, I could prepare separate PRs for them.