feat: add function ordering rule for top-level functions

[chrisophus]

Adds a new optional function rule (default: off) that enforces exported
top-level functions appear before unexported top-level functions within each
file.

The existing struct-method rule enforces export ordering for struct methods
but there was no equivalent for plain top-level functions.

init is excluded from the rule

[manuelarte]

Thanks @chrisophus for the contribution, and thanks @ldez for the review.