feature: shell macroparser

This parser is planned to perform bash style macro parsing and can be used for "styling" and "scripting" tasks within MGV. There are chances it evolves to a full featured parsing library, but currently the purpose is to "simply get the job done" :100:.

- [x] Add a simple "parser" class to replace strings within a template string
- [x] Enhance the parser to a real "bash style macro parser" - support the most important macros from MGV's view
- [x] Parse external file rules
- [x] Add a `README.md`
- [x] Add some tests
- [x] Finalize and completion before merging :exclamation: :smile:

See also 'MacGitver/MacGitverModules -> PR81'.