Add types for Prefix and PrefixMap

[cthoyt]

This PR adds the Prefix class, which allows you to do the following:

import csv
from pydantic import BaseModel, ValidationError
from curies import Converter, Prefix

converter = Converter.from_prefix_map({
    "CHEBI": "http://purl.obolibrary.org/obo/CHEBI_",
})

class Row(BaseModel):
    prefix: Prefix
    identifier: str
    name: str
    smiles: str

records = [
    {"prefix": "CHEBI", "identifier": "16236", "name": "ethanol", "smiles": "CCO"},
    {"prefix": "CHEBI", "identifier": "28831", "name": "propanol", "smiles": "CCCO"},
    {"prefix": "CHOBI", "identifier": "44884", "name": "pentanol", "smiles": "CCCCCO"},
]
for record in records:
    try:
        model = Row.model_validate(record, context=converter)
    except ValidationError as e:
        print(f"Issue parsing record {record}: {e}")
        continue