the next debugger script event, `getPossibleBreakpoints` (#37667), will
contain a single attribute `unsigned long spidermonkeyId`, so it will
have nothing in common with `addDebuggee`. this patch renames the latter
accordingly.
Testing: no behaviour changes other than the rename, so no tests needed
Fixes: part of #36027
Signed-off-by: Delan Azabani <dazabani@igalia.com>
