Add range type support

[nkantar]

parsenvy.range should expect a string comprised by two comma-separated integers—e.g., "13,42"—as its value. It should split that string, convert each part into an integer, and return a range call with them as arguments.

To-do:

• Add parsenvy.range function
  • Add the function body in the same vein as the rest
  • Add default_if_none_decorator
  • Add appropriate docstring
  • Add hints as needed
• Add tests
• Add documentation
  • Add standalone page
    • Add bash setting example
    • Add autodoc call
  • Add page to toctree in index.rst
• Make sure all the CI checks pass