Add hints to pull request template & ask for screenshots

[clairep94]

Lots of PRs related to UI are coming in without screenshots, so this PR adds hints

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines

[release-com]

Release Environments

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• p5.js-web-editor
  • app-ted60bc.release.editor.p5js.org/
  • preview-ted60bc.release.editor.p5js.org/

🔧Environment Status : https://app.release.com/public/Processing%20Foundation/env-3ceb61cc47

[raclim]

Tagging @Izzy-Snyder @runningonsideprojects, since this seems to address the suggestion to add screenshots for overall UI changes!