Extend ILLink schema

[martincostello]

Add additional ILLink properties and items related to native AoT.

Happy to move them to a different schema if needed (ILCompiler?), but there seemed to be lots of related concepts in this file already.

Resolves #205.

[mhutch]

Thanks! I have a few comments:

• EnableSingleFileAnalyzer and EnableAotAnalyzer could probably go in NetSdk.buildschema.json next to IsAotCompatible etc. IMO these are fairly general purpose as they're useful for library authors who aren't using native linking themselves.
• Please use backtick markdown for code like filenames, cli commands, etc.
• I would prefer the properties/items specific to Native AOT go in a separate file
• includeDescription isn't being used correctly, it should be a noun phrase: https://github.com/mhutch/MonoDevelop.MSBuildEditor/wiki/Schema-structure#includedescription
• Symbols that don't start with Ilc should have something about native linking in the description to help clarify when when are relevant, e.g. NativeLibrary could be changed from "Specifies a .lib file on Windows and a .a file on Unix-like systems." to "When using Native AOT, specifies a native library to be statically linked into the binary. Must be a .lib file on Windows and a .a file on Unix-like systems."

[mhutch]

Fantastic, thank you!