fix: fs.readFile: make options optional

[felixrieseberg]

Node documents options in fs.readFile/fs.writeFile operations as optional. We have done so for almost all operations, but made a mistake in fs.readFile - if you don't pass options, you'll get a TypeError that shouldn't be there.

This tiny PR fixes that.

Closes #11182

[ckerr]

LGTM.

As an aside, I notice util.isString() and util.isObject() are deprecated. Any idea what the supported way is for doing that now?

[felixrieseberg]

@ckerr Oh, good catch! Core moved those out into userland (https://github.com/isaacs/core-util-is), with the argument being that those have no place in core. I'll make another PR that goes with the module rather than the deprecated functions.