gopls/internal/lsp: add go to implementation support for function types

Improve test.

Fixes #56572

Author: a-h

gopls/internal/lsp/testdata/implementation/implementation.go

@@ -45,7 +45,7 @@ type StructWithFunctionFields struct {
 	FT FunctionType //implementations("FT", ImplementationOfFunctionType1, ImplementationOfFunctionType2)
 }
 
-func (s StructWithFunctionFields) struct {
+func (s StructWithFunctionFields) Test() {
 	s.FT("s", 0) //implementations("FT", ImplementationOfFunctionType1, ImplementationOfFunctionType2)
 }