Implement the wrenGetArgumentBool function in wren_vm.c, which retrieves a boolean value from the foreign call argument slot at the given index. The function includes assertions for valid call context, non-negative index, and argument bounds. Also add the corresponding declaration to the public wren.h header to expose the new API for foreign method implementations.
Add a new public API function `wrenReturnBool` that allows foreign function
implementations to return boolean values to Wren. The function validates
that it is called within a foreign call context using an assertion, then
writes the boolean value into the foreign call slot and clears the slot
pointer. This completes the set of return value helpers alongside the
existing `wrenReturnNull` and `wrenReturnString` functions.
Harry Lawrence