Support optional transitions

[MathiasWP]

Describe the problem

It is possible to set transitions via props when creating Svelte components:

<script>
export let transition;
</script>

<p transition:transition|local>Hi!</p>

However, they cannot be optional:

<script>
export let transition = undefined;
</script>

<p transition:transition|local>Hi!</p>

Error thrown if transition is undefined:

Describe the proposed solution

Check if transition is a function before running it.

The current implementation does not check if fn is of type "function"

Alternatives considered

Setting a default empty empty function:

<script>
export let transition = () => {};
</script>

<p transition:transition|local>Hi!</p>

However, this is a potential memory leak until this pr is merged.

Importance

would make my life easier

[umwwwelt]

maybe duplication of #7157 ;)