Enable cljs.core.specs.alpha at least at the REPL and ideally for compilation.

Rationale: For alignment with Clojure.

Note: The cljs.core.specs.alpha namespace was ported around a year ago, CLJS-2413 and has shipped since 1.10.63 as an opt-in feature.

Enabling this could slow down compilation (it would be worth checking).

Enabling this could cause certain code that compiles now to no longer compile if they contain code that doesn't pass the specs (similar to what happened in Clojure).