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:

• Add Gutenberg support to the plugin #34 (comment)
• Anchor for blocks with save that doesn't support an id prop WordPress/gutenberg#6356 (comment)
• 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 Add Gutenberg support to the plugin #34
• add a supports declaration for anchor
• add the anchor output to the pym_shortcode function.

[benlk]

After 2613c25, 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.