tweak: format yaml like weblate does

[adil192]

Following up on #299 (comment).

I want to integrate Weblate to make submitting translations easier for my contributors, but when using it, Weblate and slang fight over how the yaml files should be formatted, leading to PRs looking like this:

---

Weblate was also wrapping lines that got too long, but I can probably just disable this in the Weblate settings. I haven't added line wrapping in this PR.

[adil192]

Also some other things:

1. What do you think about adding a command that reformats all yaml files, e.g. dart run slang format?
   This way, any formatting changes from Weblate or other tools can be discarded before merge.

   dart run slang normalize already reformats all yaml files except english / the base locale, so we could alternatively just make slang normalize also format (but not normalize) the english yaml?

2. I recommend using the leading_newlines_in_multiline_strings linter rule to make these tests and their indentations easier to read, e.g.:

         final expected = '''fruits:
   - apple
   - banana
   - cherry
   ''';

   turns into

         final expected = '''
   fruits:
   - apple
   - banana
   - cherry
   ''';

[Tienisto]

Thanks! And I agree that slang normalize should also reformat the base translations.