py/make_root_pointers: Add MP_REGISTER_ROOT_POINTER parser/generator.

This adds new compile-time infrastructure to parse source code files for
`MP_REGISTER_ROOT_POINTER()` and generates a new `root_pointers.h` header
file containing the collected declarations.  This works the same as the
existing `MP_REGISTER_MODULE()` feature.

Signed-off-by: David Lechner <david@pybricks.com>

py/mpstate.h

@@ -192,6 +192,13 @@ typedef struct _mp_state_vm_t {
     // include any root pointers defined by a port
     MICROPY_PORT_ROOT_POINTERS
 
+    // Include any root pointers registered with MP_REGISTER_ROOT_POINTER().
+    #ifndef NO_QSTR
+    // Only include root pointer definitions when not doing qstr extraction, because
+    // the qstr extraction stage also generates the root pointers header file.
+    #include "genhdr/root_pointers.h"
+    #endif
+
     // root pointers for extmod
 
     #if MICROPY_REPL_EVENT_DRIVEN