Add well known PortalTarget class and examples

[myieye]

Adds PortalTarget (as discussed) as a well known class that can be used to globally customize the default portal target used by the portal action.

I also:

• Added examples of the different target approaches (see screenshot below)
• Changed the portal logic to put the portal back in its original parent if it becomes disabled (rather than doing nothing)
  I expect this was just an oversight so far. If a portal is initially disabled, it just stays in its original parent, so it seems to make sense to consider that its "disabled state". And if a user doesn't want the portal to move, they just shouldn't change the options.
  I primarily made the change, because it meant that I needed less code for the examples, but I think that it's the intuitive behaviour.

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Latest commit: d5e0ae0

The changes in this PR will be included in the next version bump.

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svelte-ux	Patch

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Name	Status	Preview	Comments	Updated (UTC)
svelte-ux	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 3, 2024 1:34pm

[techniq]

This looks great and I appreciate you adding examples to the docs, and I agree disabling should restore back to the original location, good catch. Let a few little nits then looks to merge.

[techniq]

Thanks @myieye!