Modularize canon, nquad parsing and identifierIssuer

Author: mielvds

CHANGELOG.md

@@ -26,6 +26,10 @@
     (including `None`) are no longer allowed, hence missing base IRIs in the 
     JSON-LD context are now handled outside the function call.
   - Add unittests
+ - **BREAKING**: The `IdentifierIssuer` class was moved to `identifier_issuer.py`. It's now available 
+   at `pyld.identifier_issuer`.
+ - **BREAKING**: The classes `URDNA2015` and `URGNA2012` were moved to `canon.py`. They are now available 
+   at `pyld.canon`. 
 
 ## 2.0.4 - 2024-02-16