Verbose alternation-in-optional error message

[kieran-ryan]

🤔 What's changed?

Updated error message for an alternation inside an optional from...

An alternation can not be used inside an optional
You can use '\\/' to escape the '/'

...to...

An alternation can not be used inside an optional
If you did not mean to use an alternation you can use '\\/' to escape the '/'. Otherwise rephrase your expression or consider using a regular expression instead.

⚡️ What's your motivation?

1. Improve error message consistency
   • Was You can use '\\/' to escape the '/'; while AlternativeMayNotBeEmpty message for similar is If you did not mean to use an alternative you can use '\\/' to escape the '/'
2. Improve clarity on how to fix if user intention was indeed to use an alternation and an optional. With d(a/o), fixes would be:
   • d(a\/o) - if wanted d and da/o
   • d/da/do, d(a)/do or d(a)/d(o) - if wanted d, da and do
   • or to perhaps use a regular expression

Allows to close cucumber/vscode#142 by improving clarity that message is expected behaviour - so is not re-raised.

🏷️ What kind of change is this?

• 🐛 Bug fix (non-breaking change which fixes a defect)

♻️ Anything particular you want feedback on?

Whether Otherwise rephrase your expression or consider using a regular expression instead. portion is clear or can be improved.

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

[mpkorstanje]

LGTM. But I don't see the changelog entry. Feel free to merge after adding.