BIDS 2.0: harmonize TSV columns to be singular (ATM "units" and "migrate_plural_columns")

[yarikoptic]

Closes

• https://github.com/bids-standard/bids-2-devel/issues/15
• https://github.com/bids-standard/bids-2-devel/issues/78

TODOs

• [ ] review if any other renames are due (not necessarily plural -> singular but might be done along here)
• [ ] decide on either renaming "units" to "unit" is worthwhile given that there is lots of "*Units" in sidecar fields: https://github.com/bids-standard/bids-2-devel/issues/15#issuecomment-2112984796
• [x] RECOMMEND for the user added columns to be singular
  • [ ] consider possibility for adding a check to bids-validator -- might be too tricky to be worth it since not trivial to determine plurality AFAIK
• [ ] compliment with changes to "CONTRIBUTION" guide of some kind (@effigies please remind where should we place those)
  • [ ] add a test for schema tests to ensure that nobody inadvertently adds another plural column
    • might be worth for BIDS 1.0 with whitelisting of known 2 culprits so we prevent possibly adding more
• [ ] implement migration code