Enable support for MDG syntax (Markdown with Gherkin)

[eterps]

Is your feature request related to a problem? Please describe.
If I understand correctly:

1. pytest-bdd uses the official gherkin parser for Python
2. the official gherkin parser for Python supports MDG syntax (Markdown with Gherkin)

Describe the solution you'd like
That pytest-bdd supports MDG syntax (which uses the .feature.md extension)

Describe alternatives you've considered
Staying with the plain Gherkin .feature syntax. However the markdown focused workflows when using LLM coding assistants have an excellent match with markdown flavours. There is also excellent support for syntax highlighting, formatting, linting, LSP support and other features you get for free with the markdown ecosystem.

Additional context

• gherkin/python: Support gherkin markdown cucumber/common#2103