Update PostCSS Load Config

[spencermize]

Is your feature request related to a problem? Please describe.
Hey! PostCSS Load Config now supports ESM modules as of v4.0. It'd be great to get the deps updated here so we can continue moving away from .cjs files in SvelteKit.

How important is this feature to you?
Not super important, but I think it's the last .cjs domino to fall in my repo and would unlock some things I've been looking to do for awhile.

Additional context
(https://github.com/postcss/postcss-load-config/blob/main/CHANGELOG.md#40-2022-05-23)

Thanks!

[bluwy]

postcss-load-config v4 is supported in the latest release (#520), and it's a peer dep, so you would have to bump it in your project yourselves.

[spencermize]

Ah, crap - I missed that. Thanks!