chore(gatsby): add keyword gatsby-plugin to package.json

[muescha]

Description

add keyword gatsby-plugin to package.json

i don't know if this is

Related Issues

fixes #20773 chore(packages): 404 on gatsby-cypress

follow up steps

the package need to be published to get updated on npm https://www.npmjs.com/package/gatsby-cypress

[LekoArts]

Hi!
The keyword was left out intentionally as it shouldn't be listed in the plugins search. It's a helper package for Cypress and not a Gatsby plugin that you'll put into gatsby-config.js.

[muescha]

i expected it also in the plugins, because it adds functionality to gatsby somehow.

otherwise the gatsby-cypress is nowhere visible on the docs page. maybe add something to https://www.gatsbyjs.org/docs/end-to-end-testing/ ?