Use mfm-js for MFM parsing (#7415)

* wip

* Update mfm.ts

* wip

* update mfmjs

* refactor

* nanka

* Update mfm.ts

* Update to-html.ts

* Update to-html.ts

* wip

* fix test

* fix test

src/prelude/tree.ts

@@ -1,36 +0,0 @@
-import { concat, sum } from './array';
-
-export type Tree<T> = {
-	node: T,
-	children: Forest<T>;
-};
-
-export type Forest<T> = Tree<T>[];
-
-export function createLeaf<T>(node: T): Tree<T> {
-	return { node, children: [] };
-}
-
-export function createTree<T>(node: T, children: Forest<T>): Tree<T> {
-	return { node, children };
-}
-
-export function hasChildren<T>(t: Tree<T>): boolean {
-	return t.children.length !== 0;
-}
-
-export function preorder<T>(t: Tree<T>): T[] {
-	return [t.node, ...preorderF(t.children)];
-}
-
-export function preorderF<T>(ts: Forest<T>): T[] {
-	return concat(ts.map(preorder));
-}
-
-export function countNodes<T>(t: Tree<T>): number {
-	return preorder(t).length;
-}
-
-export function countNodesF<T>(ts: Forest<T>): number {
-	return sum(ts.map(countNodes));
-}