Add highlighting for gherkin language

Hello all, support for gherkin (gherkin language syntax highlighting) would **be** great!

Especially like in Cucumber feature files:
https://cucumber.io/docs/gherkin/reference/
Code examples and keywords are explained on linked web site.

thanks a lot!

No repository file or test is named in the issue. Start by reading the linked Cucumber Gherkin reference and then inspect the syntaxhighlighter repository to find how language syntax highlighting is registered. Done means Cucumber-style Gherkin feature files receive appropriate highlighting.

Written by the indexing model from the issue text.