Expose on:tick from ForceSimulation to allow for dynamic simulation tuning

[regexident]

A common technique used to keep force-directed graphs with collision forces butter-smooth is collision easing:
Slowly ramping up the collide force's strength from an initial 0 to the desired end value by updating it on each tick().

This way the layout has a chance to very quickly settle on a "pretty good yet possibly overlapping" layout before turning up the collide force for nudging the nodes to their final positions, while resolving any remaining overlaps.

By adding tick event like …

type TickEvent = CustomEvent<{
    alpha: number;
    // Any other values worth providing here?
}>;

… and a corresponding on:tick to ForceSimulation one would be able to implement collision easing (or any other dynamic simulation fine-tuning for that matter) along these (pseudo-code) lines, by coupling the weighted strength to the simulation's current alpha value (or any other timing value available to the component itself):

<script lang="ts">

import * as d3 from 'd3';

import { writable, type Writable } from 'svelte/store';

const collideStrengthTarget: number = 1.0;

let forces: Writable<Record<string, d3.Force<any, any>>> = writable({
    link: d3.forceLink(links).distance(0).strength(1.0),
    charge: d3.forceManyBody().strength(-50.0),
    collide: d3.forceCollide().strength(0.0),
    x: d3.forceX(),
    y: d3.forceY()
});

function handleTick(event: TickEvent) {
    const weight = d3.easeQuadInOut(1.0 - event.alpha);
    // I'm assuming here that ForceSimulation's force objects have reference semantics:
    forces.update((forces) => forces.collide.strength(weight * collideStrengthTarget));
}
</script>

<ForceSimulation
    {forces}
    ...
    on:tick={handleTick}
>
    <!-- ... -->
</ForceSimulation>

[techniq]

This makes sense to me, but I would love to see an example of this in action (or even a PR 😁)

Updating forces should reactively update (see Force Group example which does something similar). I also have a good bit of additional reference examples in issue #139, but I haven't seen this kind of tick/easing.