Constant definition with all caps should trigger a custom error message

[Nick Seagull]

Something like this:

neo> MY_CONSTANT = 10
error:
  It looks like you're trying to define a constant in all caps. In NeoHaskell, constants are defined in `camelCase`,
  as names that start with uppercase letters are reserved for types.

  Read more about constants in the docs: https://neohaskell.io/docs/essentials/constants