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

[benlk]

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.

[benlk]

After https://github.com/INN/pym-shortcode/commit/2613c2557e797cd797b44da73f15a7f58d49832e, we have a save method that does store some things in the post_content directly, so it's worth rechecking to see if we can grab that.

[benlk]

in progress on 35-anchor-redux, but still no idea how to get the anchor value passed to the shortcode, even when trying to pull it from the post_content.