GRSciColl - add identifier type ISIL

[ManonGros]

I was made aware of the existence of the International Standard Identifier for Libraries and Related (ISIL) https://slks.dk/english/work-areas/libraries-and-literature/library-standards/isil which is used to catalogue a number of museums across the world.
I would like to be able to import data from ISIL as well as link museums to ISIL when the identifiers are available.

Here are examples of ISIL identifiers: https://slks.dk/english/work-areas/libraries-and-literature/library-standards/examples

I think we should also enforce the format they suggest. It is described here: https://slks.dk/english/work-areas/libraries-and-literature/library-standards/structure. Also pasting below the first part of the description (please check the link for more details):

The ISIL is a variable length identifier. The ISIL consists of a maximum of 16 characters, using digits (Arabic numerals 0 to 9), unmodified letters from the basic Latin alphabet and the special marks solidus (/), hyphen-minus (-) and colon (:). Latin letters modified with one or more diacritics and letters from alphabets other than Latin cannot be used in the ISIL.

Each ISIL identifier shall be unique when normalized to the repertoire of characters specified in ISO/IEC-10646-1.

ISIL country code and non-country code prefixes are case insensitive. The case sensitivity of the library code shall be decided at the national level. For instance, in Finland FI-Ht and FI-HT are two different ISILs. In the UK, GB-UkLoRHS and GB-uklorhs are two variants of the same ISIL and identify the same organization.

An ISIL is made up by two components: a prefix and a unit identifier, in that order, separated by a hyphen-minus. The hyphen-minus is a mandatory character in the ISIL string.