fix(core)!: changed the order expected from open

[dk949]

This is inline with documentation and the way diff expects it's return values.

This is a breaking change, however, I think it will mostly affect hooks, not the core functionality. Namely this will change the value passed for winnr to post_open and may cause block_end to not be called.

This also only people who were using open as a function, rather than a string, and have already swapped the two values.

Also updated type of files in README. This is not a breaking change as using the argument as a string[] will have resulted in an error.