Plot distributions with colour instead of line types

[jamesmbaazam]

Description

This PR closes #786 by using the color aesthetic instead of linetype to plot distributions in plot.dist_spec.

Initial submission checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have tested my changes locally (using devtools::test() and devtools::check()).
• [ ] I have added or updated unit tests where necessary.
• [ ] I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• [x] I have followed the established coding standards (and checked using lintr::lint_package()).
• [x] I have added a news item linked to this PR.

After the initial Pull Request

• [ ] I have reviewed Checks for this PR and addressed any issues as far as I am able.