Block anchors

[main-kun]

Add a plugin to support {#my-anchor} syntax for anchors detached from content

[moki]

let's go for the syntax: {% anchor anchor-id %}

regexp: ^{%[^\S\r\n]*anchor[^\S\r\n]+[\w-]+[^\S\r\n]*%}

since we want to make leaf block node plugin, let's match only paragraphs with that syntax exclusively.

fair example would be:

transform/src/transform/plugins/checkbox/checkbox.ts

Lines 8 to 14 in d0d2651

	function matchOpenToken(tokens: Token[], i: number) {
	return (
	tokens[i].type === 'paragraph_open' &&
	tokens[i + 1].type === 'inline' &&
	tokens[i + 1].content.match(pattern)
	);
	}