…s for internal links

Render link labels as SVG `use` elements when the label text is an id pointing to the element provided via `svgDefs` to the container. This allows the link labels to use SVG icons instead of simple text

Consolidate utility exports in `packages/ts/src/index.ts` by creating a new `utils/index.ts` file that re-exports all utility modules. This simplifies the main entry point and makes it easier to discover and use the available utility functions.

- Ensures the `fitView` method waits for the `_layoutCalculationPromise` to resolve before calling `_fit` to prevent potential errors.

- Adds `onZoomStart` and `onZoomEnd` callbacks to the `GraphConfigInterface` to allow users to hook into the start and end of the zoom behavior.
- Implements the `_onZoomStart` and `_onZoomEnd` methods to call the respective callbacks when the zoom behavior starts and ends.

- Adds an optional `nodeIds` parameter to the `fitView` method that allows the caller to specify which nodes to fit the view to.
- Updates the `_fit` method to filter the nodes to the specified `nodeIds` if provided, or use all nodes if not.
- This enables users to focus the view on a subset of nodes, rather than always fitting the entire graph.

…dify the layout