Add a PR template with the What's Changed entry convention

[bernardnormier]

icerpc-csharp recently introduced a PR description convention (icerpc/icerpc-csharp#4838): every PR description ends with a ## What's Changed entry section that feeds the "What's Changed" section of the GitHub release notes. Each entry is either:

• an Area: <area> block with one bullet per user-visible change, written from the user's perspective, or
• None — <short reason>. for changes with no user-facing impact.

We are already asking for this section in slicec PRs (e.g. #802, #803), but slicec has no .github/pull_request_template.md, so contributors have no skeleton or rules to follow.

We should add a PR template to this repository that mirrors the icerpc-csharp template, adjusting the example area names to fit slicec.

[InsertCreativityHere]

Instead of doing it by "area" I'm going to do it by "crate".
So going forward every "What's Changed" should either say: none, slicec, or slice-codec.
I don't think any good PR should ever meaningfully change both of those at once.

A related thought: this might not be necessary? This repo already has labels for slicec and slice-codec, and I use them everywhere.
In the future, maybe we could rely on these tags instead of parsing it from the What's Changed entry? A thought for the future though.
For now, I'll just keep parity with icerpc-csharp.