Make ns-syntax-command emit a `let b:clojure_syntax_keywords` dict

Commit d776ecac5528031a0eb5f293c572c1e35aacd75a of vim-clojure-static
introduced the b:clojure_syntax_keywords dictionary, which allows users
and plugins to augment the core syntax groups for a buffer in a way that
persists after a syntax reload.
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