Custom prefix (X-) for arbitrary columns in .tsv files

[yarikoptic]

This is a twin for

• Custom prefix (X-) for arbitrary metadata in .json files #80

which is IMHO more obvious/easier to address. Relates to

• Formalize "life-cycle"/movement and placement of metadata #85

in a step towards unification.

Should be easy to automatically migrate on BIDS 1 -to- 2.

[effigies]

Just a note that it is often convenient for fields in JSON and columns in TSV to have names that are valid identifiers, which are typically /[A-Za-z0-9_]+/.

[yarikoptic]

so ___ or 0 is valid too? Should we aim to constrain more, e.g. /[A-Za-z][A-Za-z0-9_]*/ if we are to not enforce camel-vs-snake but at least for it to start with a letter?

[effigies]

At least javascript is happy to have "0" or "__" as keys, but sure, there is a safer regex.