[libkwineffects] Expose getting/setting activeFullScript to scripted effects

Summary:
Getter is exposed as a property on scripted effect in a way that hides
pointers from the scripting side.

Setter is implicitly handled as a property of newly created animations
and holds the activeFullScreenEffect whilst any of them are active. Like
existing effects it remains up to the effect author to avoid the
problems of multiple full screen effects. The RAII lock pattern is
somewhat overkill currently, but it's the direction I hope we can take
EffectsHandler in next API break.

BUG: 396790

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This patch is against the QJSEngine port, though it's not conceptually a
requirement.

Test Plan: Unit test

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14688