Improve releasing script

[LouisCAD]

• [x] 1. Make releasing script check the correctness of the CHANGELOG heading (I wrote "Versions" instead of "Version" and it led to empty body of pre-filled GitHub release, should never happen again)
• [ ] 2. Offer to open urls directly at other places where it suggests to click.
• [ ] 3. Pre-fill PR as it works too after clicking the green button.
• [ ] 4. Ask for next version upfront, so the 2 last steps Commit 'prepare next dev version' and Push, at last get executed without user input (and therefore, can't be forgotten).