Deprecate Parser::splitWhitespace() / Parser::createAssocArgs()

Deprecate Parser::splitWhitespace() / Parser::createAssocArgs()

These methods are not used anywhere in deployed code.

Parser::createAssocArgs() is used in Extension:DataTable2, but it should
just copy the implementation from Parser, it's very little code.

Code search:
https://codesearch.wmflabs.org/deployed/?q=splitWhitespace%7CcreateAssocArgs&i=nope&files=&repos=

Bug: T236810
Change-Id: I4e6a39e2ecf7a3a568e26ad1d8ce1166a44a5ad9
(cherry picked from commit d7cd12e81ff6622bb5e14980297f926a3dec0984)