feat: export core primitives for third-party plugin components

WithProps and its types (ElementOptions, Attach, LeafletElement,
LeafletElementConstructor, LeafletAddEventListener/LeafletRemoveEventListener),
the shared-props.ts fragments (pathProps, latLngProps, tileLayerProps,
urlProp, style, getBounds, Positioned/Styleable/Sourced/Bounded), and all
of event-types.ts are now part of the public API surface, not just
internal to this package's own components.

The registration protocol (instanceof Layer/Popup/Tooltip against
Leaflet's own base classes) and the icon-changed protocol were already
plugin-friendly by design -- a third-party layer, control, or icon
component built on Leaflet's own class hierarchy integrates with what's
here without any special-casing. This closes the one real gap: those
same primitives weren't reachable from the package root, only via
undocumented dist/core/* deep imports. A plugin author can now build a
component the same way this package's own components are built.

src/index.ts

@@ -21,6 +21,13 @@
 // can go anywhere.
 export * from './core/register.ts';
 export * from './core/props.ts';
+// WithProps, the prop-fragment helpers, and the event-map fragments are the
+// same toolkit our own components are built from -- exported so a
+// third-party plugin component can be built the same way, rather than
+// reaching into dist/core/* directly (undocumented, not a stable contract).
+export * from './core/with-props.ts';
+export * from './core/shared-props.ts';
+export * from './core/event-types.ts';
 export { LeafletMap } from './components/leaflet-map.ts';
 export { LeafletControlLayers } from './components/leaflet-control-layers.ts';
 export { LeafletLayerGroup } from './components/leaflet-layer-group.ts';