feat: add option to set default PR merge strategy

[lentscode]

I usally use the "Squash" merge on PRs, it's quite annoying having to change the merge strategy for each PR.

I propose an option that sets the default merge strategy from the three available.

[juliusmarminge]

This is already available.

#8088 added:

• remembering the last merge method you pick across PRs/sessions
• a per-project Default merge method in project settings (merge / squash / rebase)

Closing as resolved. Reopen if that still isn't covering what you need.