Revert "Disable diagnostics in the preview window"

[sharwell]

Reverts #67030 pending design review.

[CyrusNajmabadi]

Reasoning for disabling were given in the original PR. Even if this is useful (very questionable), it has to warrant its cost. RIght now it does an enormous amount of work (like running source generators) just to show a potential squiggle for a few seconds. We can reenable this once we show we can do so in a way that doesn't cost as much.

This feature is also not going to be there once we switch onto pull-diagnostics (expected to go into dogfooding next week), and is not supported as part of LSP. So any time this would be back on would be short lived. Given that, and the poor perf, turnign off to give people direct perf benefits for actually important scenarios (lightbulb) is more critical.

[sharwell]

This still needed to go to a team design review in advance of the merge.