Avoid warning issues with peer dependencies

[delight]

Allow svelte-preprocess ^6 to avoid warnings when using the latest dependencies.

 WARN  Issues with peer dependencies found
.
└─┬ svelte-check 3.8.1
  └─┬ svelte-preprocess 5.1.4
    └── ✕ unmet peer postcss-load-config@"^2.1.0 || ^3.0.0 || ^4.0.0 || ^5.0.0": found 6.0.1

it's a dependency, not a peer dep

[dummdidumm]

We can't do this because it's a dependency, and that dependency requires node 18, while we claim node 16 still. But we'll do a new major of svelte-check soon where we can include this.

[dummdidumm]

Closing in favor of #2401