Enable more analyzers for code style consistency and prevent other common mistakes

[manfred-brands]

We should enable both Microsoft.NetAnalyzers and StyleCop.Analyzers to align the code and keep it consistent.

Some files still contain tabs. We should get a warning and fix that. Some are even mixed mode (Options.cs)
Fields only set in the constructor should be marked readonly.
Properties only set in the constructor should only have a get, not a (private) set
Structures types should be marked readonly where possible.
Classes should be sealed if not meant to be extensible.
Use expression bodies for properties if single-lined.
...

We can enable rules as you see fit.

[CharliePoole]

@manfred-brands I'm in favor of adding the Microsoft.NetAnalyzers. Do you want to take a shot at setting it up with some set of rules to start with? All the stuff you mention is fine for me.

To a large extent, however, I don't like to have too many stylistic rules. To be clear, my experience, before I retired, was working with groups of highly competent developers who were able to read and code using a variety of styles without confusion. That may bias me and I'm happy to talk about it.