feat: add `fork`

[ppeeou]

Fixes #70

@darky I'd like to get feedback from you What do you think about the expression and behavior?

• [x] sync
• [x] async
• [x] concurrent
• [ ] error propagation

[darky]

@ppeeou Great, thanks!

[ppeeou]

@hg-pyun Since all the checkboxes have not been completed yet, I changed it to draft !