New construct: assignment sigil '='.

With tests.
Escaping constructs '~' and '{~}' in Positron::DataTemplate
Documentation of assignment and escaping
Release v0.0.5

  - Added assignment construct '=' to DataTemplate.
  - Added escaping constructs '~' and '{~}' to DataTemplate.
Tweak to dist.ini: Git::Commit message

This looked like 'vv0.0.5'. Changed (hopefully) to 'Release v.0.0.5'.