[API] Add LinkConnector - replaces connecting_links (#726)

### LinkConnector

Replaces the minimal-change effort of `connecting_links` with a more
reliable implementation.

- Subscribable, event-based API
- Uses browser-standard `e.preventDefault()` to cancel `before-*` events
- Uses standard `state` POJO - can be proxied without issue
- Structures code and separates concerns out of `LGraphCanvas`
- Link creation calls can now be made from anywhere, without the need
for a rewrite
- New canvas sub-components now live in `src/canvas/`

### Rendering

- Skips link segments by setting a `_dragging` bool flag on the LLink or
Reroute
- Moves some previously nested code to event listeners, configured in
the `LGraphCanvas` constructor

### Deprecation

`LGraphCanvas.connecting_links` is now deprecated and will later be
removed.

Until it is removed, to prevent breaking extensions it will continue to
be set and cleared by a legacy callback. The contents of this property
are ignored; code search revealed no exentsions actually modifying the
array.

src/interfaces.ts

@@ -113,6 +113,13 @@ export interface LinkNetwork {
   getNodeById(id: NodeId): LGraphNode | null
 }
 
+/**
+ * Locates graph items.
+ */
+export interface ItemLocator {
+  getNodeOnPos(x: number, y: number, nodeList?: LGraphNode[]): LGraphNode | null
+}
+
 /** Contains a cached 2D canvas path and a centre point, with an optional forward angle. */
 export interface LinkSegment {
   /** Link / reroute ID */
@@ -131,6 +138,9 @@ export interface LinkSegment {
    */
   _centreAngle?: number
 
+  /** Whether the link is currently being moved. @internal */
+  _dragging?: boolean
+
   /** Output node ID */
   readonly origin_id: NodeId | undefined
   /** Output slot index */