feat: enable Github Issue Forms

[willmartian]

Resolves #5414

Changes:

Replaced the existing Markdown issue templates with GitHub issue forms. I believe that the forms make it easier to contribute to p5.js, which furthers its mission of improving access.

To preview the new functionality, feel free to open an example issue in my fork.

Screenshots of the change:

User view upon selecting "new issue":

User view upon selecting "Existing Feature Enhancement":

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[limzykenneth]

This looks better than I expected and I think really would make not just authoring an issue but also for us going through an open issue much easier!

A few changes inline.

@outofambit @Qianqianye Would you like to have a look at this as well?

[outofambit]

thanks for doing this @willmartian, I'm really excited to get this in! left a few comments, let me know what you think? :D

[willmartian]

Thanks for the reviews all! And the patience, I stopped looking at GitHub over the holiday :)

I think I implemented all of the requested changes--let me know if there is anything else @outofambit

[limzykenneth]

Thanks! I'll merge this in a couple days if there are no further comments.

[limzykenneth]

Merged. There's a few things I may want to tweak but for now, let's give this a go!