Adding GB 2.0 compatibility and new filter for devs to update blocks.

[blronaldhuereca]

This PR does the following:

1. Adds GenerateBlocks 2.0 compatibility (using generateblocks/text)
2. Adds a new filter to update the list of third-party headline blocks programmatically (this is so you don't have to keep updating the plugin to support other block suites).

Filter in the code:

/**
 * Filter to add supported third party blocks.
 *
 * @param array $supported_third_party_blocks The array of supported third party blocks.
 * @return array The modified array of supported third party blocks.
 */
$supported_third_party_blocks = apply_filters(
	'simpletoc_supported_third_party_blocks',
	$supported_third_party_blocks
);

[mtoensing]

Elegant change. Thank you