Guest language registry. Maps language names to configurations.
A guest language is: a name, a file extension, an optional prelude,
and optional rewrite rules.

(register! :pattern {:extension ".pat"
                     :prelude-file "langs/pattern/core.meme"
                     :rules-file  "langs/pattern/rules.meme"})

(resolve-lang "app.pat")  → :pattern
(lang-config :pattern)     → {:extension ... :prelude ... :rules ...}