fix: detect missing closing brace in block parsing to prevent crash

The compiler previously relied on `match(TOKEN_RIGHT_BRACE)` in the while loop condition, which consumed the brace and made it impossible to detect a missing one. Changed to peek-based loop termination and added explicit `consume` call for the closing brace, ensuring proper error reporting when '}' is absent. Added regression test for issue #429.

test/regression/429.wren

@@ -0,0 +1,6 @@
+// The missing "}" was not detected by the compiler and reported as an error,
+// so it then tried to run the resulting erroneous bytecode and crashed.
+
+// expect error line 6
+{
+System