[LORO-369] Simplify Tree about loro

[joakim]

I read on Discord that you'd like some feedback on the Tree API.

Here's how I would prefer to work with trees:

const doc = new Loro();

// ... load from persistence ...

const root = doc.getTree('root') // LoroTree

// 'data' instead of 'meta', as a getter?
root.data.set('foo', 123);

// create a child directly on the parent (returns the child node)
const child = root.createNode(); // LoroTree
child.data.set('bar', true);

// create child of child
const child2 = child.createNode();

// move to other node (append if ordered)
child2.moveTo(root) // argument: LoroTree

// move before/after other node (anywhere in the tree)
child2.moveBefore(child) // argument: LoroTree
child2.moveAfter(child)

// reorder children just the same
child.moveAfter(child2)

// get all children
console.log(root.children) // LoroTree[child2, child]

assertEquals(child.parent, child2.parent)
assertEquals(root.children[0], child2)
assertEquals(root.lastChild, child)

// delete node
child2.delete()

[zxch3n]

Done