gherkin: Fix gherkin/javascript/examples so Gherkin loads in browser

[aslakhellesoy]

Summary

Assign Gherkin to window if it's defined

Details

The example in gherkin/javascript/examples threw an error in the browser, complaining that Gherkin was not defined

Motivation and Context

Needed to use Gherkin in a browser.

Related to cucumber/gherkin#204

How Has This Been Tested?

Running the example manually

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.

[lock]

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