feat(gherkin): added custom flavor registry

[sashokbg]

🤔 What's changed?

Added the ability to register custom token matcher for custom "gherkin flavors". "generateMessages" and "makeSourceEnvelopes" will try to match the file extension / media type with the according custom flavor and use the appropriate matcher.

⚡️ What's your motivation?

I would like to implement a custom "flavor" using asciidoc much like the one with markdown that already exists in the repository.

Also implementing such feature adds an SPI-like capabilities for everyone to implement whatever they like as "flavors" and keeps the open / close principle.

related to #10

🏷️ What kind of change is this?

• :zap: New feature (non-breaking change which adds new behaviour)

♻️ Anything particular you want feedback on?

📋 Checklist:

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

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[sashokbg]

bump any comments/discussion are greatly appreciated @aslakhellesoy