Update API information for 2020.2.0a18

[citizenmatt]

This PR updates the API information to Unity 2020.2.0a18.

It should be that simple, but Unity have introduced a method that takes a List<string> and it's the first generic argument we've had to handle and it breaks everything. We were storing parameter and types as IClrTypeName, and using this interchangeably with C# types (e.g. when generating method declarations for code completion).

However, IClrTypeName can only hold, as the name suggests, CLR type names. This means open generics, rather than closed generics. And it does it in CLR format, not C# format. So it can hold System.Collections.Generic.List'1 (that should be a backtick, but markdown), but it can't hold System.Collections.Generic.List1[[System.String]]`. We also can't generate a method declaration from the open generic CLR type name, but need to convert it to a closed generic type first.

• Update API information to Unity 2020.2.0a18. Closed generics are written in CLR form into the api.xml file and parsed into open generic + type parameter types when reading
• API information is converted into IType using TypeFactory. This gives us proper type information, including closed generics
• We create a lot of instances of types for "derives from" checks, completion + inspections, so the types are now cached per solution in a new class - KnownTypesCache
• Added extra undocumented event functions