Used colons for separating the line and character number in prose formatter

[reduckted]

PR checklist

• Addresses an existing issue: Use colons for error reporting for lines/char instead on brackets #3861
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Changed the prose formatter to use the format:

file:line:column - message

instead of:

file[line, column]: message

Is there anything you'd like reviewers to focus on?

Nope.

CHANGELOG.md entry:

[enhancement] prose formatter uses line:column instead of [line, column]

[JoshuaKGoldberg]

Simple, clean, and approved in theory by giladgray. LGTM!

Thanks for this!