Issue forms with YAML templates

Originally suggested in https://github.com/Leaflet/Leaflet/issues/7704#issuecomment-981410604.

Issue forms can help people create better issues, which is helpful to maintainers.

Public repo to try the new templates out: https://github.com/Malvoz/test-issue-form-templ/issues/new/choose.

File as many issues as you want there, the repo will be deleted after it has served its purpose. The content in that repo is intended to replace the contents of .github/ISSUE_TEMPLATE/, if accepted.

If you think this is a good idea, please discuss any changes you’d like to see in the templates.

More info:

• Issues forms beta for public repositories
• Creating issue forms
• Syntax for GitHub’s form schema

/cc @mourner, @IvanSanchez, @Falke-Design, @jonkoops, @johnd0e, @perliedman, @yohanboniface, @cherniavskii

[mourner]

@Malvoz looks great, let’s do it!

[Malvoz]

• Added needs triage labelling in https://github.com/Malvoz/test-issue-form-templ/commit/5bff026ff53bbdbb0928c5ad333c065e0efacf39.
• Changed to use a link for “Leaflet version” in https://github.com/Malvoz/test-issue-form-templ/commit/fd539621a161cbdc7a46d2fed0193d9ad2f07c04
• Added example for Environment in its description, in https://github.com/Malvoz/test-issue-form-templ/commit/b1ce1126ba5a4b44f483583cd196ac15e5550528, https://github.com/Malvoz/test-issue-form-templ/commit/c02bf6d5cc5f798aba69478c0385e2f1db6aa46d. Please take a look at the bug report to see if the example looks good or not.