Add a character table file option to enable user-provided character table(s)

Usage: --character-table-file FILE_PATH

Where FILE_PATH is a user-provided file with 256 glyphs to be used as the character table.

This allows users to create/use their own character tables without requiring support for hardcoded tables.

To improve flexibility, I'd suggest making the file JSON (or perhaps JSON5 to enable comments) and should be an array/list with 256 strings. Each string could be more than one byte to enable Unicode glyphs but should be just 1 "character".

Also add support for an environment variable that sets the character table file path.

Current character tables could be refactored to this format internally, although it's a relatively non-trivial change since they currently use an enum and code to generate characters as needed.