WIP: add trimming support

[zivillian]

I started to prepare the library for trmming according to documentation.

My goal was to be able to publish my project (which uses YamlDotNet) trimmed without getting a warning. I know, that there was some previous work in #740 and #758, but even though I used a static context it still produced warnings because YamlDotNet is not marked as trimmable.

Now I'm a bit stuck, because the docs recommend to

Avoid annotating virtual methods or interface methods.

But there are still ~60 warnings, but all remaining methods are either overrides or interface implementations.

Since I've spent a few hours, I didn't want to just throw it away and instead created this draft. Maybe someone has an idea how to continue.

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I've deleted some methods and files, because they were unused, internal and produced additional trim warnings - so deleting was easier than annotating them.