Add function `stripPrefix`

The intention is to strip away a common prefix and return the relative FilePath, if and only if there is a common prefix.
Possible function signature:
```haskell
stripPrefix ::
-- | Root directory. May be relative or absolute.
FilePath ->
-- | FilePath to strip away from the root directory.
FilePath ->
-- | FilePath relative to the root directory or Nothing.
Maybe FilePath
```

Some example calls:
```haskell
>>> stripPrefix "app" "app/File.hs"
Just "File.hs"

>>> stripPrefix "src" "app/File.hs"
Nothing

>>> stripPrefix "src" "src-dir/File.hs"
Nothing

>>> stripPrefix "." "src/File.hs"
Just "src/File.hs"

>>> stripPrefix "app/" "./app/Lib/File.hs"
Just "Lib/File.hs"

>>> stripPrefix "/app/" "./app/Lib/File.hs"
Nothing -- Nothing since '/app/' is absolute

>>> stripPrefix "/app" "/app/Lib/File.hs"
Just "Lib/File.hs"
```

Motivation: I often want to strip away some directory from another FilePath if and only if the given FilePath is part of the directory. This is some combination of `makeRelative` and checking, whether the given FilePath is in the given directory.

Other programming languages, such as Rust (https://doc.rust-lang.org/std/path/struct.Path.html#method.strip_prefix) also implement a similar function in the standard library for Path manipulation.

Is such a function useful to others as well?
Or maybe, this function is trivial to express with the given API already?