Implement initial support for `if` expressions in the compiler, including new `TOKEN_IF` and `TOKEN_ELSE` tokens, `CODE_JUMP` and `CODE_JUMP_IF` bytecodes for conditional branching, and a `makeNull()` function in the VM to represent the value returned when an if-condition is false without an else clause. The `emit()` function now returns the index of the emitted bytecode to enable patching jump offsets. Also add a `nullClass` to the VM and a `OBJ_NULL` object type, along with a `dumpCode()` debugging helper and new test files for block syntax and if-expression behavior.
