make extended cell attributes extendable

[jerch]

Coming from #2503. Also related #2904.

Extended attributes on cells offer a simply way of extending terminal buffer functionality for other purposes. Currently it is not possible to extend them in a flexible way to be used for custom data from addons.

TODO: Come up with an easy to interface to extend extended attributes from addons. Bonus goal - make this type safe even for runtime extensions (hard one with TS' static type system).

[Tyriar]

Related: #2919

[jerch]

@Tyriar Yeah the CellMarker idea is somewhat related. But I am not sure, whether to go that path at all - in the image addon I cancelled that idea and went with a viewport scan, which turned out to be fast enough for that purpose. CellMarker would certainly speedup those tasks, but would penalize the buffer logic at other ends.

[jerch]

Somewhat obsolete, thus closing.