patch 9.0.1741: No type checking in interfaces

Problem: No type checking in interfaces
Solution: Implement member type check in vim9 interfaces

Most of the code is a small refactoring to allow the use of a where_T
for signaling the type mismatch, the type checking itself is pretty
simple.

Improve where_T error reports

Let the caller explicitly define the kind of location it's referring to
and free the WT_ARGUMENT enum from its catch-all role.

Implement type checking for interface methods

Follows closely the logic used for type-checking the members.

closes: #12844

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: LemonBoy <thatlemon@gmail.com>

src/testdir/test_vim9_class.vim

@@ -1576,6 +1576,61 @@ def Test_class_implements_interface()
     endclass
   END
   v9.CheckScriptFailure(lines, 'E1349:')
+
+  lines =<< trim END
+      vim9script
+
+      interface One
+        static matching: bool
+        static as_any: any
+        static not_matching: number
+      endinterface
+      class Two implements One
+        static not_matching: string
+        static as_any: string
+        static matching: bool
+      endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1406: Member "not_matching": type mismatch, expected number but got string')
+
+  lines =<< trim END
+      vim9script
+
+      interface One
+        def IsEven(nr: number): bool
+      endinterface
+      class Two implements One
+        def IsEven(nr: number): string
+        enddef
+      endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1407: Member "IsEven": type mismatch, expected func(number): bool but got func(number): string')
+
+  lines =<< trim END
+      vim9script
+
+      interface One
+        def IsEven(nr: number): bool
+      endinterface
+      class Two implements One
+        def IsEven(nr: bool): bool
+        enddef
+      endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1407: Member "IsEven": type mismatch, expected func(number): bool but got func(bool): bool')
+
+  lines =<< trim END
+      vim9script
+
+      interface One
+        def IsEven(nr: number): bool
+      endinterface
+      class Two implements One
+        def IsEven(nr: number, ...extra: list<number>): bool
+        enddef
+      endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1407: Member "IsEven": type mismatch, expected func(number): bool but got func(number, ...list<number>): bool')
 enddef
 
 def Test_call_interface_method()