Make semantic token mapping more accurate

[ijklam]

Make semantic token mapping more accurate. And add a new modifier "Extension" to make user can make themes that can distinguish extension methods out from intrinsic methods.
See ionide/ionide-vscode-fsharp#2114

Basically, changes are:

1. ReferenceType → Class
2. MutableVar → Variable + Mutable
3. MutableRecordField → Property + Mutable
4. UnionCaseField → Parameter
5. Method → Method (not Function)
6. ExtensionMethod → Method + Extension

[baronfel]

Independent of any actual theme changes, it's always lovely to have more precise scopes! Thanks for taking the time to dig into this area.

[TheAngryByrd]

Thank you!!