[FR] Add replaceShallow function to matchers

The `matchers` module have functions that operate either just on the given node (`extract`), or in the node and its children (`findall`, `extractall`). However for `replace` there's no such distinction.

One could mimic that behavior by first calling `m.matches(node, matcher)`, but that would be much less performant.

Ideally these two functions would be named `replace` and `replaceall`, to be consistent with the rest of the api, but I do not know what policies you have in place with regards to backwards compatibility.