Use `util.inspect.custom` symbol if available.

Use `util.inspect.custom` symbol if available.

Ports the `buffer-inspect` test from Node core.
Adds a `Buffer.prototype[util.inspect.custom]` method, which is an alias
to `Buffer.prototype.inspect`. Node already doesn't have an `.inspect`
method on Buffers anymore, but since this module has to work in browsers
that do not have Symbols, it seems better to keep it around.

In Node, this will use the builtin `util.inspect.custom` symbol. In the
browser, it will use `Symbol.for('util.inspect.custom')`. The browser
version of `util` will also use the `inspect-custom-symbol` module in
the near future.

If https://github.com/nodejs/node/pull/20857 gets merged,
`Symbol.for('util.inspect.custom')` will be used everywhere and the
dependency on `inspect-custom-symbol` could probably be dropped.

The motivation for this is API parity and the fact that Node is
removing support for the old `.inspect` method:
https://github.com/nodejs/node/pull/20722