Use Gutenberg's native `anchor` support for setting the pym parent element ID

We didn't do this in the initial implementation of Gutenberg blocks in #34 because, at the time, it appeared as if there was no way to use Gutenberg's `anchor` property with a block that was dynamically rendered.

For more history, see the following comments:

- https://github.com/INN/pym-shortcode/pull/34#issuecomment-417355256
- https://github.com/WordPress/gutenberg/issues/6356#issuecomment-417176119
- https://github.com/WordPress/gutenberg/pull/8154/files, the current version of which is https://wordpress.org/gutenberg/handbook/block-api/#supports-optional

Elements of this ticket:

- [ ] remove the shim introduced in #34
- [ ] add a `supports` declaration for `anchor`
- [ ] add the `anchor` output to the pym_shortcode function.