Anchors are lost when their position is assigned a Node value

[eemeli]

Describe the bug
When a node with an anchor is replaced by a different node, the anchor is lost and any aliases referring to it will fail to stringify properly.

To Reproduce

const doc = YAML.parseDocument('foo: &a A\nbar: *a\n')

// This modifies the existing Scalar
doc.set('foo', 'B')
String(doc)
// → 'foo: &a B\nbar: *a\n'

// This replaces the Scalar
doc.set('foo', doc.createNode('C'))
String(doc)
// → Uncaught Error: Alias node must be after source node

Expected behaviour
Either anchors should be automatically re-assigned, or it should bepossible/easy to ensure that happens.

Versions (please complete the following information):

• yaml: all

Additional context
This is effectively a different expression of the "Fix self-referential aliases on custom collections" todo item on #203, as a good solution to this will also provide a good solution to that.

[eemeli]

The original buggy part of this is getting fixed by #248, so I'm going to close this for now. The wontfix label is here for the surprising-behaviour part, because I don't think there's a good API change for it that wouldn't either be really clumsy, or be actually less surprising.