feat: show aliases in usage

[gkc]

• [x] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

What I did

• Add a flag to allow option alias(es) to be output by ArgParser's usage
• The default behaviour remains the same - i.e. aliases are not printed in usage
• Kind of like what is requested in https://github.com/dart-lang/args/issues/240 but for ArgParsers' Options, not CommandRunners' Commands

How I did it

• usage_test.dart:
  • Add a test to verify that aliases are shown for options if requested
  • Add a test to verify that aliases are not shown for options by default
• usage.dart: make _longOption also show aliases, if any, if showAliases is true
• added showAliases parameters in various places so that we can preserve current behaviour as default
• updated CHANGELOG.md