feat: add defer_save, cancel_save, immediate_save trigger options

[primeapple]

This fixes #8

[primeapple]

Just from the codestructure I'm happy now.
I'll try using it from next week on and see if there are any bugs.

@okuuva feel free to have a look over it.

I also added some TODOs that might be good for later improvements.

[okuuva]

Excellent work @primeapple! I'll take it into daily use as well to spot any potential problems but LGTM already :)

[okuuva]

And I do agree with the TODOs. I wouldn't maybe drop those in this MR just yet just to keep things as atomic as possible. And I need to comb through the code a few more times since I've also wondered what's the point with those seemingly unused things. Probably remnants from the rewrite Pocco81 did in July 2022 but need to triple check.

[primeapple]

And I do agree with the TODOs. I wouldn't maybe drop those in this MR just yet just to keep things as atomic as possible.

I'll remove the todos and open another PR afterwards to add them, maybe even removing the potential unused code.

Thanks for the review :)

[primeapple]

I have some trouble because it doesn't seem to be possible to set timers to buffer variables, see here: https://www.reddit.com/r/neovim/comments/123y048/cannot_convert_userdata_when_trying_to_set_a/

I might just use the global_vars variable after all...

[primeapple]

Ok. I open this as "Ready for review". It works quite well on my machine now, you are save to try it out @okuuva :)

[okuuva]

Haven't had time to test this thoroughly since I was away for the weekend and been sick since I got back. But couldn't find any obvious problems with the short time I had to test it, the code looks good to me and I trust you've been testing it yourself during the development much more rigorously than I have. Exceptional job!

[primeapple]

Ok, thank you!
Let's observe how it behaves in the future ;-)