ItemizedResource should be a fixed SBS grid

[rickwierenga]

ABC.. 1234...

this will simplify serialization and access etc.

the current model is more flexible, which is annoying sometimes

relevant:

• https://discuss.pylabrobot.org/t/toml-instead-of-py-files-for-labware/500
• https://discuss.pylabrobot.org/t/thoughts-on-having-a-compact-save-format-for-a-deck-a-resource-that-isnt-just-the-built-in-serialize/534/

good first issue: requires some design, if codex could one shot it I would have done that instead of writing issues