reflect: implement CanInterface and fix string Index()

This commit fixes two related issues:

 1. CanInterface was unimplemented. It now uses the same check as is
    used in Interface() itself.
    This issue led to https://github.com/tinygo-org/tinygo/issues/3033
 2. Allow making an interface out of a string char element.

Doing this in one commit (instead of two) because they are shown to be
correct with the same tests.

testdata/reflect.go

@@ -415,6 +415,9 @@ func showValue(rv reflect.Value, indent string) {
 	if rv.CanAddr() {
 		print(" addrable=true")
 	}
+	if !rv.CanInterface() {
+		print(" caninterface=false")
+	}
 	if !rt.Comparable() {
 		print(" comparable=false")
 	}