docs: Improve issue template.

[smjonas]

I think using yaml instead of markdown is way better for user experience because it allows things like required fields, checkboxes and non-modifiable text :D I got a bit of inspiration from Neovim's issue template. I also removed the "Describe the bug" section because there was a lot of overlap with the "To reproduce" and "Additional context" fields. Also, "Actual behavior" seemed to be missing which is why I added that instead.

I created a repo for you where you can test this out: https://github.com/smjonas/test/issues/new/choose

Let me know what you think!

[NoahTheDuke]

Fwiw, this is much harder to use on mobile and straight up doesn't work in the GH app. (It loads the browser.)

[smjonas]

When I open the link on mobile I am simply redirected to the website (probably this is not implemented in the app yet). Filling it out there works just fine for me.

[kylechui]

Thanks for the PR! I might defer this change until forms are out of beta; at the very least until I finish pushing out some other features.

[smjonas]

Btw, I don't think this feature will be out of beta anytime soon :D It's been in beta since at least July of last year. Both Vim and Neovim have since adopted it.