Add issue template for language proposals

[glopesdev]

As the language evolves we want to make sure we are both as inclusive as possible, and as structured as possible, so that we do not miss the opportunity to document details of new language features as they are being investigated.

Here we introduce a template for creating language proposals with the goal of helping our language discussion and documentation.

Fixes #1738

[PathogenDavid]

IMO this is premature over-structuring. I think issue templates (particularly lengthy ones) are actually less inclusive - and the LDM ones are in fact intentionally exclusive. While they do establish clear expectations for what needs to be communicated, they create a huge barrier to entry and convey a sense of "Only those who are worthy can post things here." (This one doesn't even convey a sense of it, it outright says that.)

The templates used by the C# LDM fit their needs well, but their needs are significantly different from ours, their community is significantly larger than ours, and their internal processes are significantly different from ours.

I'm all for refining our development processes, but I don't think this is the path to something successful or helpful.

---

If you're dead set on this, IMO it's a mistake to redirect people to discussions for informal proposals. We should reserve them for questions and announcements. Informal proposals will be drowned out otherwise.

We should also have clear messaging that submitting bug reports as issues is desired/acceptable.

We should use issue forms instead of old-school markdown templates.

[banchan86]

Agree with @PathogenDavid here, I think the template as is, is overly formal and restrictive, I would be hesitant to fill this out.

I do support introducing a basic issue template that doesn't impose too high a barrier of entry. It might also nudge people to consider the issue more in depth. Something with just the headings (Description, Motivation, Possible Solution, Alternatives) and a basic question for each. I recall seeing something done for either the harp-tech issues or discussions, but I can't find it now.

I also think we shouldn't direct people to discussions, I like keeping issues neat within their own repository.