Add NestedWith check

[easink]

Reports a with that does not start at the top of a function body: one buried inside an if, unless, case, cond, for, receive, or another with. Such a with splits a chain of pattern matches across two levels of control flow, which hides its failure paths and makes the else of either block ambiguous to a reader.

def, defp, defmacro, defmacrop, fn and defmodule open a scope, so a with at the top of one of their bodies is never reported, however deeply the scope itself is nested. defmodule counts because a module is routinely wrapped in a compile-time if Code.ensure_loaded?(Dep), which is conditional compilation rather than control flow around the with. try is neither scope nor block and is passed through: wrapping a chain in try/rescue does not branch it, and a try has no else to confuse with the one belonging to the with.

The traversal is an explicit recursion rather than Credo.Code.prewalk/3 because the flag it carries has to be reset per branch, and reset entirely on the way into a scope.

Opt-in for now, so it lands in :disabled.