Add xsd: and linkml: as built-in prefix names.

We add the two following prefix names:

* xsd: http://www.w3.org/2001/XMLSchema#
* linkml: https://w3id.org/linkml/

to the list of built-in prefix names. They are intended to be used when
defining "extension slots" (as the value of the `type_hint` slot, to
define the expected type of the slot), so for convenience we make them
built-in.

src/docs/spec-intro.md

@@ -27,3 +27,5 @@ Throughout the specification, the following IRI prefix names are used:
 | semapv      | https://w3id.org/semapv/vocab/ |
 | skos        | http://www.w3.org/2004/02/skos/core# |
 | sssom       | https://w3id.org/sssom/ |
+| xsd         | http://www.w3.org/2001/XMLSchema# |
+| linkml      | https://w3id.org/linkml/ |