micropython/usb/usb-device/usb/device/core.py: Add alternate setting support plus a few fixes and improvements.

[HLammers]

Summary

• Added bAlternateSetting as argument to Descriptor.interface, because it is used for USB MIDI 2.0
• Fixed the loop reserving other string indexes used by built-in drivers while len(strs) < builtin_driver.str_max: to while len(strs) < builtin\_driver.str\_max - 1: (row 169) because str_max equals to one more than the highest string descriptor index value used by any built-in descriptor according to the docs and added a comment that this loop is probably unnecessary or even wrong
• Removed import time (not used)
• Removed unused/duplicate _Device.config argument config_str
• Some code optimisations (in particular to the Buffer class)

Testing

• Tested with micropython/usb/examples/device/midi_example.py
• Tested the Buffer class with micropython/usb/usb-device/usb/device/tests/test_core_buffer.py
• Tested with a soon to be made public multi-port MIDI module draft which adds port names to strs

Trade-offs and Alternatives

None

[HLammers]

The updated version of core.py is used in this project, which also shows that it is working: https://github.com/orgs/micropython/discussions/17640