Implemented extern functions

src/token/Tokenize_test.go

@@ -25,13 +25,14 @@ func TestFunction(t *testing.T) {
 }
 
 func TestKeyword(t *testing.T) {
-	tokens := token.Tokenize([]byte("assert if import else loop return struct switch"))
+	tokens := token.Tokenize([]byte("assert if import else extern loop return struct switch"))
 
 	expected := []token.Kind{
 		token.Assert,
 		token.If,
 		token.Import,
 		token.Else,
+		token.Extern,
 		token.Loop,
 		token.Return,
 		token.Struct,