chore: refactor signal types

[Rich-Harris]

Another piece of #10594. While working on that PR I found it made a huge difference to update the various signal types:

• use interface instead of type (interface is less forgiving)
• rename SourceSignal to Source, etc
• make Derived extend Source, rather than deriveds sharing a type with effects, since they have much more in common with each other
• have a Value type which is a union of Source and Derived, and a Reaction type which is a union of Derived and Effect
• avoid using the Signal type (which is a union of all three) unless necessary

There's more to do still (renaming some variables, replacing the one character signal.x style properties with something a bit less cryptic, getting rid of unused Derived properties) but that can happen in follow-up PRs.

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