Add insensitive case character class

[mingodad]

Would be nice to have character class case insensitive [a-z]i for grammars where identifiers are case insensitive (SQL, Pascal, ...).

Here is an example on peggy playground https://peggyjs.org/online.html (also implemented here https://github.com/mingodad/peg):

start = name_char+ 
name_char =
     [a-z0-9$_]i* [ \t\n]

Input:

one
Two
One