Explicit IUPAC tables

[ahwagner]

Currently, we link to the Wikipedia IUPAC tables, and previously the genome.jp IUPAC tables which have some slight differences (termination and gap codes). We should be explicit about what we accept and what it means.

Create IUPAC tables in spec, citing one of these sources. Update Sequence regex in schema accordingly.

[larrybabb]

We need to take the Regex expressions in our code and explicitly document them in our Residue section of the docs to support the consistent reference (whether that be the wikipedia link or the genome.jp link or both above).

Look at the recently updated biotuils version and make sure our vrs2.0 code and docs reference that. We should also keep these updated as we build new releases of VRS beyond 2.0.