Improved way to create metavars coloured strings

Fixes #125.
Fixes #127.

[x] Implemented the suggestion from @hamdanal for the creation of the metavars strings and Spans.
[x] The implementation avoids using regex and only produces the coloured strings once for all help parts (usage and argument description).
[x] A few minor modifications were implemented from the original suggestion.
[x] Added test for metavars that span within multiple lines if terminal window is not big enough (see #125).
[x] 100% coverage on all tested python versions.

hamdanal / rich-argparse