Add support for 856 field indicator 2 values 3 and 4

[Galen Charlton]

This is closely related to bug 1992827, but is not quite a duplicate.

In October 2022, the MARC21 powers that been accepted the following MARC proposal: https://www.loc.gov/marc/mac/2022/2022-07.html

The proposal changes the definition of the second indicator of the 856, to wit:

blank: tweaks definition to apply to the resource as a whole
0: makes a stronger claim that the resource as a whole described by the bib can be found at the link
1: makes a stronger claim that an online version of the whole resource is available at the link
2: narrows the definition of a related resource
3: new value; means that the link is a _part_ of the resource, e.g., "volume 1", "volume 2", etc.
4: new value; means that the link goes to an online version of _part_ of the resource

Implications I see are:

- indicator values 3 and 4 should trigger the display of the link with a suitable label (though please, not literally "Version of component part(s) of resource")
- as 856 fields get updated over time, there may be more reason to include ind2 = blank in displays
- both staff and OPAC would need to be adjusted

I note that I am a bit dubious that the distinction that MARC tries to draw between the bib record describing an electronic resource vs. it describing a resource that also happens to have an online version is going to be hugely meaningful to almost all patrons.

[Elaine Hardy (ehardy)]

Perhaps leave any descriptive text for indicator 4 out and rely on any descriptive note in another subfield to define the specific URL? I think it would be more meaningful for patrons to just see "Table of contents" than
Version of component part(s) of resource: Table of contents.

This might mean adding the |3 but I think it would be much more helpful