common/collections: Fix type checking in Append

The fix introduced in Hugo `0.49.1` had an unintended side-effect in the `Append` func used in both `append` and `.Scratch.Add`.

This commit fixes that by loosen/fixing the type checking so concrete types can be appended to interface slices.

Fixes #5303

common/collections/append_test.go

@@ -43,7 +43,13 @@ func TestAppend(t *testing.T) {
 			tstSlicers{&tstSlicer{"a"},
 				&tstSlicer{"b"},
 				&tstSlicer{"c"}}},
+		{testSlicerInterfaces{&tstSlicerIn1{"a"}, &tstSlicerIn1{"b"}},
+			[]interface{}{&tstSlicerIn1{"c"}},
+			testSlicerInterfaces{&tstSlicerIn1{"a"}, &tstSlicerIn1{"b"}, &tstSlicerIn1{"c"}}},
 		// Errors
+		{testSlicerInterfaces{&tstSlicerIn1{"a"}, &tstSlicerIn1{"b"}},
+			[]interface{}{"c"},
+			false},
 		{"", []interface{}{[]string{"a", "b"}}, false},
 		// No string concatenation.
 		{"ab",