chore: add increment/decrement operators and test suite to parser and tokenizer

Implement prefix and postfix ++/-- operators in the tokenizer (Inc/Dec token types) and parser (factor/unary functions) with correct pre/post semantics. Add increment_decrement_test.rc covering all four operator forms. Update Makefile to include the new test file in TESTS, add run-increment-decrement-test target, and fix .PHONY and TARGET dependency on dirs.

tests/increment_decrement_test.rc

@@ -0,0 +1,30 @@
+int main() {
+    int a;
+    int b;
+
+    a = 5;
+    b = a++;
+    printf("a = %d, b = %d\n", a, b);
+
+    a = 5;
+    b = ++a;
+    printf("a = %d, b = %d\n", a, b);
+
+    a = 5;
+    b = a--;
+    printf("a = %d, b = %d\n", a, b);
+
+    a = 5;
+    b = --a;
+    printf("a = %d, b = %d\n", a, b);
+
+    a = 5;
+    a++;
+    printf("a = %d\n", a);
+
+    a = 5;
+    a--;
+    printf("a = %d\n", a);
+
+    return 0;
+}