Idea: Allow underscore-prefixed identifiers

In Haskell, prefixing an identifier with an underscore is allowed. E.g. the following snippet compiles in Haskell, but not in Curry:

_test :: Int
_test = 3

Anonymous.curry:3:2 Error:
    Unexpected token identifier `test'
   | 
 3 | _test :: Int
   |  ^

Such identifiers could be useful for a variety of reasons:

In Haskell, a common pattern is to name the implementation of a function (possibly taking additional parameters) _f and the exported, public interface function f
In Haskell, underscore-prefixed identifiers can be used to silence warnings about unused parameters. This could be useful if a parameter is explicitly to be marked as unused, but still assigned a name. For example:

const :: a -> b -> a
const x _y = y

would not emit a warning in Haskell. There is an additional challenge that this example shows, however:

Allowing underscore-prefixed identifiers is a breaking change, since the current Curry parser reads the function above as

const x _ y = y

(and thus throws a type error). If we were to implement a change that allows such identifiers, e.g. the following snippet would no longer be valid Curry:

const2 :: a -> b -> c -> a
const2 x _y = x

Same goes for the (currently) equivalent spelling

const2 :: a -> b -> c -> a
const2 x __ = x

While such formatting is usually considered unidiomatic, if we were to implement this feature, we should detect such cases and emit a descriptive error message along with a fix to rewrite the function as:

const2 x _ _ = x