feat: add support for if/else statements

src/frontend/lexer.rs

@@ -64,6 +64,8 @@ impl<'src> Lexer<'src> {
 
         match &self.source[start..self.cursor] {
             "fn" => TokenKind::Fn,
+            "if" => TokenKind::If,
+            "else" => TokenKind::Else,
             "return" => TokenKind::Return,
 
             "i8" => TokenKind::I8,
@@ -203,12 +205,16 @@ mod test {
     #[test]
     fn identifiers() {
         assert_eq!(
-            tokenize("HELLO _hello _0@"),
+            tokenize("HELLO _hello _0@ fn if else return"),
             vec![
                 Token::new(TokenKind::Identifier, "HELLO", Span::new(0, 5)),
                 Token::new(TokenKind::Identifier, "_hello", Span::new(6, 12)),
                 Token::new(TokenKind::Identifier, "_0", Span::new(13, 15)),
                 Token::new(TokenKind::Invalid, "@", Span::new(15, 16)),
+                Token::new(TokenKind::Fn, "fn", Span::new(17, 19)),
+                Token::new(TokenKind::If, "if", Span::new(20, 22)),
+                Token::new(TokenKind::Else, "else", Span::new(23, 27)),
+                Token::new(TokenKind::Return, "return", Span::new(28, 34)),
             ]
         )
     }