feat: support prettier v3

[dummdidumm]

Prettier v3 changed some APIs to be asynchronous. This is an easy fix, we just add await to some methods which is useless but harmless for v2 invocations.

The more involved problem is that the v3 Prettier API is incompatible with v2, and likewise you can't use prettier plugins that support v2 with Prettier v3 and vice versa. This means we need to add additional code to keep things working when someone has Prettier (or the plugin) installed but not the other (the plugin or Prettier). The current code does ignore the user's Prettier version if it's v3 but prettier-plugin-svelte is not found - it falls back to v2 of both Prettier and the plugin. It falls back to v2 for now because based on installs Prettier v2 is still far more popular so that's the safer choice.
There's one problem though: What if someone has other plugins in their node modules but not the prettier plugin? In this case formatting could be done with Prettier v2 using plugins (from the user) that only support v3, and that fails. I don't see a good way around this, but I would also hope this does not happen often.

Thoughts @jasonlyu123 ?

[ghostdevv]

Perhaps we could have a "best guess" at prettier version and have a way to override it if it guesses wrong?

[jasonlyu123]

I also can't think of ways around it. Probably easier to just show a message alert. But the problem is, can we reliably identify if the problem is caused by a plugin incompatible with a prettier version? Or we can only provide a troubleshooting hint.

[dummdidumm]

I fear we can't identify if the problem is caused by a plugin, so showing a hint everytime the formatting goes wrong could only be very generic. It would also be shown if everything is ok on the plugin-side but someone just has invalid code, which would be a lot of false positives. I'm therefore against a hint, but we should already log something to the output tab so if people look there they should get a lead.

[dummdidumm]

tested:

• prettier v3 + plugin v3 -> works
• prettier v2 + plugin v2 -> works
• prettier v2 + no plugin -> works
• prettier v3 + no plugin -> works

Should be safe. I needed to adjust the code some more because Prettier v3 has removed the plugin search feature and so we need to load the config to see if plugins: ['prettier-plugin-svelte'] is in there. Another review would be appreciated.

(will look into if we can test this in the test suite)