fs: Add file descriptor support to *File() funcs

These changes affect the following functions and their synchronous counterparts:

 * `fs.readFile()`
 * `fs.writeFile()`
 * `fs.appendFile()`

If the first parameter is a String (i.e. a path), the original implementation is used. Otherwise, a file descriptor is assumed. File descriptor ownership is never taken from the user.

The documentation was adjusted to reflect these API changes. A note was added to make the user aware of file descriptor ownership and the conditions under which a file descriptor can be used by each of these
functions.

Tests were extended to test for file descriptor parameters under the conditions noted in the relevant documentation.
All of them, or just the ones related to this PR?