[Core] Remove gherkin from public and internal api

[mpkorstanje]

Summary

Removes all gherkin related classes from the public and internal APIs
This ensures that a future Gherkin update will only impact a limited
section of cucumber

Because the information from pickles, and features still need to be
accessed it wrapped in our own API. This also makes it possible to
provide access to a steps keyword. This in turn removes the
**keyword** from the snippet suggested events allowing them to
be used directly.

By including the snippet suggestion each time a test fails because a
step is undefined test runners that group output per test case can
clearly document which steps need to be implemented for each scenario.

This will allow users to use Cucumber JUnit without including the
summary plugin. Allowing a bit more configuration to be replaced by
defaults.

Screenshots (if appropriate):

IDEA collects stdout output and groups displays it per unit test:

Maven does the same thing per JUnit test case:

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.