Implemented extern functions

src/token/Kind.go

@@ -66,6 +66,7 @@ const (
 	___KEYWORDS___                    // <keywords>
 	Assert                            // assert
 	Else                              // else
+	Extern                            // extern
 	If                                // if
 	Import                            // import
 	Loop                              // loop

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,

src/token/identifier.go

@@ -19,6 +19,8 @@ func identifier(tokens List, buffer []byte, i Position) (List, Position) {
 		kind = If
 	case "else":
 		kind = Else
+	case "extern":
+		kind = Extern
 	case "import":
 		kind = Import
 	case "loop":