[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/NodeSlot.ts

@@ -1,4 +1,4 @@
-import type { CanvasColour, ConnectingLink, Dictionary, INodeInputSlot, INodeOutputSlot, INodeSlot, ISlotType, IWidgetInputSlot, Point } from "./interfaces"
+import type { CanvasColour, Dictionary, INodeInputSlot, INodeOutputSlot, INodeSlot, ISlotType, IWidgetInputSlot, Point } from "./interfaces"
 import type { LinkId } from "./LLink"
 import type { IWidget } from "./types/widgets"
 
@@ -83,9 +83,9 @@ export abstract class NodeSlot implements INodeSlot {
 
   /**
    * Whether this slot is a valid target for a dragging link.
-   * @param link The link to check against.
+   * @param fromSlot The slot that the link is being connected from.
    */
-  abstract isValidTarget(link: ConnectingLink | undefined): boolean
+  abstract isValidTarget(fromSlot: INodeInputSlot | INodeOutputSlot): boolean
 
   /**
    * The label to display in the UI.
@@ -270,10 +270,8 @@ export class NodeInputSlot extends NodeSlot implements INodeInputSlot {
     return this.link != null
   }
 
-  override isValidTarget(link: ConnectingLink | undefined): boolean {
-    if (!link) return true
-
-    return !!link.output && LiteGraph.isValidConnection(this.type, link.output.type)
+  override isValidTarget(fromSlot: INodeInputSlot | INodeOutputSlot): boolean {
+    return "links" in fromSlot && LiteGraph.isValidConnection(this.type, fromSlot.type)
   }
 
   override draw(ctx: CanvasRenderingContext2D, options: Omit<IDrawOptions, "doStroke" | "labelPosition">) {
@@ -306,10 +304,8 @@ export class NodeOutputSlot extends NodeSlot implements INodeOutputSlot {
     this.slot_index = slot.slot_index
   }
 
-  override isValidTarget(link: ConnectingLink | undefined): boolean {
-    if (!link) return true
-
-    return !!link.input && LiteGraph.isValidConnection(this.type, link.input.type)
+  override isValidTarget(fromSlot: INodeInputSlot | INodeOutputSlot): boolean {
+    return "link" in fromSlot && LiteGraph.isValidConnection(this.type, fromSlot.type)
   }
 
   override isConnected(): boolean {