gatsby-link: add forwardRef

[ryanflorence]

This allows other components that manage focus (like a dropdown) to manage focus on a gatsby link

[ryanflorence]

Flew this one in blind through the github edit interface as I haven't actually set up gatsby locally on my new machine, looks like all the test blew up, will get back to this soon.

[DSchau]

Hi Ryan 👋

Thanks for the PR. Yeah, as you mentioned, you'll probably want to get local version of Gatsby set up. I'd recommend checking out the How to Contribute docs if you haven't already. One of the unit tests is actually failing and the e2e tests are failing because of my mistake 😅

Particularly noteworthy is the gatsby-dev-cli section, which will be super helpful here. So what I'd recommend doing will look something like this:

1. Set up gatsby-dev-cli and globally install it
2. Create a test repo using this feature (doesn't need to be in this PR at all)
3. yarn watch --scope=gatsby,gatsby-link
4. Run gatsby-dev --set-path-to-repo [your-path-to-local-gatsby]
5. Run gatsby-dev --scan-once --packages gatsby gatsby-link
6. Run yarn develop to test the gatsby-link component 🎉

Let me know if you have any questions, hopefully this helps!

[pieh]

Is this still needed now that we don't wrap link component in Location HoC? ( https://github.com/gatsbyjs/gatsby/pull/8061/files ). Users can now do this in their code, right? I didn't use forwardRef so not sure here

[m-allanson]

I think it's ok to close this? When #7546 is merged forwardRef handling will be added, but for now I think we're ok without. I put together a little test here: https://focus-gatsby-link.netlify.com/

Please re-open if I've got this wrong 👍

[m-allanson]

Oh there's a newer PR for adding forwardRef also: #9892