Fix: use DestroyImmediate when disposing LifetimeScope outside Play Mode

[Acharad]

Fixes #814

Hit the same issue @Nitero described in #814 — calling Dispose() on
a LifetimeScope outside Play Mode throws, because it always calls
Destroy(), which only works while the game is actually running. Ran
into this myself while writing an Editor test, and figured I'd fix it
properly instead of working around it locally.

Dispose() now checks Application.isPlaying and calls
DestroyImmediate(gameObject) when it's false. Play Mode behavior
(Destroy) is unchanged.

Didn't add an automated test — this repo's only test assembly
(VContainer.Tests.asmdef) isn't restricted to the Editor platform,
so Unity runs it under PlayMode, where Application.isPlaying is
always true and this branch would never actually be exercised.
Verified manually instead. Happy to add a proper Editor-only test
assembly in a follow-up if that'd be useful.

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