allow to write alternative implementations in config file

lean has `implemented_by` https://github.com/leanprover/lean4/blob/1f9bba9d398e207ebbb62696b84502ad764d39dc/src/Init/Data/Array/Basic.lean#L533

and I would like to implement optimizations like this https://github.com/purescript/purescript-json/pull/9

proposal

```purs
module PureScript.Backend.Optimizer.Directives.Defaults where

defaultDirectives :: String
defaultDirectives =
"""
...

JSON.fromBoolean implemented_by unsafeCoerce
JSON.fromInt implemented_by unsafeCoerce
JSON.fromString implemented_by unsafeCoerce
JSON.fromJArray implemented_by unsafeCoerce
JSON.fromJObject implemented_by unsafeCoerce
JSON.Array.signleton implemented_by unsafeCoerce [unsafeCoerce x]
JSON.Internals.toArray implemented_by unsafeCoerce
JSON.Internals.fromArray implemented_by unsafeCoerce
JSON.Internals.empty implemented_by unsafeCoerce []
"""
```

Alternative - implement js backend for lean :)

Start with the referenced Lean implemented_by declaration and the purescript-json pull request, then inspect PureScript.Backend.Optimizer.Directives.Defaults and how defaultDirectives is consumed. Done requires an agreed configuration syntax and behavior for alternative implementations, along with the corresponding backend support and verification.

Written by the indexing model from the issue text.