Capitalize first letter of PR title for the changelog

[Rémi Verschelde]

Some users write their PR titles without starting with a capital letter. We've recently started editing them to fix the formatting, so we could consider this solved as much as we want it to be.

But opening this issue anyway to discuss whether we'd want to do some auto-capitalization for the changelog.

Examples that would benefit from it:

Worth noting for those, they had a "GDScript: " prefix that was removed, so this would need to be taken into account in the capitalization logic.

But other cases wouldn't:

(This last one arguably warrants removing the doc: prefix, I'll open another issue #6.)