Close #1124: only assume string interning in cljs

[frenchy64]

Close #1124

I think the string interning is for JS dead-code elimination purposes. This doesn't work in bb and I'm not sure if other platforms guarantee string interning. Since there isn't much (any?) benefit, perhaps we could use the safer = on other platforms.

I'm not sure how to test CLJS and verify the js bundle size.

$ bb -Dmalli.registry/mode=strict
Babashka v1.12.194 REPL.
Use :repl/quit or :repl/exit to quit the REPL.
Clojure rocks, Bash reaches.

user=> (System/getProperty "malli.registry/mode")
"strict"
user=> (require 'malli.core)
nil
user=> (malli.registry/-schemas (malli.registry/custom-default-registry))
{}
user=>

$ clj -J-Dmalli.registry/mode=strict
Clojure 1.11.4
user=> (System/getProperty "malli.registry/mode")
"strict"
user=> (require 'malli.core)
nil
user=> (malli.registry/-schemas (malli.registry/custom-default-registry))
{}

[ikitommi]

makes sense, thanks!