Adapt the specification issue template to an issue form

[PathogenDavid]

@glopesdev asked me to adapt the existing specification proposal template to GitHub issue forms to see how that might look for this repo.

You can get an idea of how it works by submitting an issue to my fork.

For the most part this is a 1:1 adaption of the existing issue template to issue forms.

Since I added a link to GitHub Discussions for early informal specifications, I also tweaked the title of the template to clarify it's meant for formal proposals.

Using the form template does affect the appearance of submitted issues, but only superficially.
You can compare my test issue with #44 to see the differences.

One unfortunate quirk of issue forms is that the proposal status shows up as a section you can modify. As far as I could find, there's not a better way to represent static markdown comment that should just be added to the issue. (I plan to give GitHub feedback about this.)

Unresolved questions

• Right now the "informal" proposal link submits to the Open-ended discussion category, should we add a proper Proposal category and link to that? (And maybe delete the default GitHub ones to cut down on the noise)
• Should we have a template for bug reports? (Even if it's just a blank issue template)
  • You can still submit normal markdown issues using the "Open a blank issue" link on the template chooser, but it's a bit hard to notice. (It's possible for us to remove that link if we wanted to only have the templates.)
• I marked "Summary" and "Motivation" as required to enforce the final sentence of the intro paragraph. GitHub won't let you submit the issue without filling these in. Do we want to remove that sentence since it's now self-evident?
  • Alternatively, do we think enforcing it is too aggressive? (I could see how it'd be annoying when submitting a placeholder issue during a meeting, although you could just type TBD in them or whatever.)