breaking: play transitions on mount by default

[dummdidumm]

closes #11280

During testing this I noticed two things:

• in the added test, without |global the intro transition does not play within the component. Is that expected, because it's not inside any block (like an if block)?
• HMR prevents playing the intro transition for some reason. We can investigate that separately if we don't want to hold up this PR

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[Rich-Harris]

Is that expected

Yes — it's the same as if foo in {#if foo}{#if bar}<div in:fade>...</div>{/if}{/if} turned truthy — the transition wouldn't play

HMR prevents playing the intro transition for some reason

There's logic to disable transitions when the component is swapped out. Updated it so that it doesn't apply on first render