Add support for additional control subfields ($2, $3, $u)

[kirkhess]

I initially noticed this situation with the 510 field, which supports u but not 3 (then drops u in bibframe2marc...)

By looking at the specs to determine which subfields are and are not supported, I looked at control subfields (https://www.oclc.org/bibformats/en/controlsubfields.html) to see where they were not supported. Here's a list that I think you can easily add support by just using the existing templates from the control subfield stylesheet:

$2

• v034/s2
• v047/s2
• v377/s2
• v380/s2
• v382/s2
• v506/s2
• v520/s2
• v856/s2

$3

• v050/s3
• v380/s3
• v510/s3
• v524/s3
• v544/s3
• v563/s3
• v758/s3

$u (URI)

• v370/su

Literal - the following are ones I think could be supported only if you change the conversion to not use a literal value:

• v046/s2
• v046/s3
• v052/s2
• v250/s3
• v383/s2
• v383/s3
• v384/s3
• v524/s2