Create the structure for input handling

Create the structure for input handling

Create prototypes of some needed types (token, parse tree) and
create the structure of the input processing with empty functions.

Also add a grammar for minishell inside a comment in parsing.h.
This is a simplification of the grammar of Shell programming language
as it is laid out inside
The Open Group Base Specifications Issue 7, 2018 edition
Chapter 2. Shell Command Language
found on site
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html