Allowing negative literals in the Elm frontend.

[nwokafor-choongsaeng]

Describe the bug
The Elm frontend currently interprets negative literals as a function call to the negate function with the literal number as an argument.

{-| 
the line below produces an IR that looks like  
foo = negate 2
-}
foo: Int
foo = -2

To Reproduce
Steps to reproduce the behavior:

1. Create a Morphir Elm project
2. in one of the modules, add the example above
3. run morphir-elm make
4. run the develop tool and use the X-Ray view to inspect the IR for the function.

Expected behavior
Expects a negative literal to be created.

Additional context
Add any other context about the problem here.