[Strings] Support missing :char operations

[RickBarretto]

One example of it is whitespace?:

$> whitespace? " "
=> true

$> whitespace? ' '

══╡ Type Error ╞══════════════════════════════════════════════════════════════════════════════════════════════ <repl> ══
  Erroneous type found

  Cannot call function:
      whitespace? string ◄

  Wrong argument (at position 1):
      :char

  Expected:
      :string

Functions that would support it:

• [] prefix? 'a' "Abacate"
• [] suffix? 'n' "Spain"
• [] whitespace? ' '