[feature] Implement all remaining XPath and XQuery 3.1 Functions & Operators

[joewiz]

According to https://github.com/eXist-db/documentation/blob/master/src/main/xar-resources/data/xquery/xquery.xml#L111-L149, the current list of functions not implemented in eXist is:

• [x] fn:collation-key#1, #2 - https://github.com/eXist-db/exist/pull/4435
• [x] fn:default-language#0 - https://github.com/eXist-db/exist/pull/4360
• [x] fn:document-uri#0 - https://github.com/eXist-db/exist/pull/3644
• [ ] fn:element-with-id#1, #2
• [x] fn:format-integer#2, #3 - https://github.com/eXist-db/exist/pull/4366
• [ ] fn:nilled#0
• [x] fn:path#0, #1 - https://github.com/eXist-db/exist/pull/4402
• [ ] fn:round#2
• [ ] fn:transform#1
• [ ] fn:uri-collection#0, #1
• [ ] map:find#2
• [ ] map:merge#2 - already in place, but not spec compliant - so we will hold on checking this off until the merge of this PR: https://github.com/eXist-db/exist/pull/3738

This is a master issue to track progress toward this goal.

The definitions can be found at https://www.w3.org/TR/xpath-functions-31/

[yamahito]

+1 for uri-collection()!