Add Appstream metadata file

[blt-r]

For #5624.

In the future when new release is out we should add

<release version="23.12" date="2004-12-07" />

or maybe even something like

<release version="23.12" date="2004-12-07">
  <description>
    <p>"X = 'extend_line_above'" is now the default!</p>
  </description>
  <url>https://helix-editor.com/news/release-23-12-highlights/</url>
</release>

[the-mikedavis]

Could you add a bullet for updating this file between these two bullets in the release docs?

helix/docs/releases.md

Lines 7 to 8 in 17acadb

	* Merge the changelog PR
	* Tag and push

[blt-r]

Maybe it would be a good idea to link the spec somewhere? Just for future reference

[the-mikedavis]

Would the appdata file allow comments? I think it would be good to link it near the top of the file if comments won't mess up the appdata parsing

[the-mikedavis]

Should the whole list of prior releases be included too? Or just starting from 22.12?

[blt-r]

We might include every release and link to the blog post. While not necessary, it's probably a good idea

[blt-r]

I added the ones that have blog posts

[blt-r]

Would the appdata file allow comments? I think it would be good to link it near the top of the file if comments won't mess up the appdata parsing

I mean in the releases.md. I'll add a commit