Handle Symbol Unit Custom Names

[Andrew-Collins]

Hi @set-soft,

Firstly, thank you for all your hard work on kibot and associated tools.

The intention of this pull request is to add handling of custom unit names, as described in:
https://docs.kicad.org/7.0/en/eeschema/eeschema.html#creating-and-editing-symbols
(can't link to the exact subsection, search for 'Set Unit Display Name')

Kibot (v1.6.3) gives the following error if a symbol with a custom unit name is used:
ERROR:Unknown symbol attribute `[Symbol('unit_name'), '<UNIT NAME>']` (kibot.gs - gs.py:731)

The sexpr formatting isn't described in: https://dev-docs.kicad.org/en/file-formats/sexpr-schematic/
But is pretty basic: (unit_name "<UNIT NAME>")

I have added handling of this field, and stored it in the LibComponent object.

A minimum reproducable project can be found here:
https://github.com/Andrew-Collins/unit_name_mrp.git

[set-soft]

Thanks for the patch @Andrew-Collins !
Note that I also added the code to store the attribute when creating a variant of the schematic. Also note that the changes are in the dev branch.