Extends peerDependencies for latest postcss-load-config@6

[delight]

 WARN  Issues with peer dependencies found
.
└─┬ svelte-check 3.8.0
└─┬ svelte-preprocess 5.1.4
└── ✕ unmet peer postcss-load-config@"^2.1.0 || ^3.0.0 || ^4.0.0 || ^5.0.0": found 6.0.0

Before submitting the PR, please make sure you do the following

• It's really useful if your PR relates to an outstanding issue, so please reference it in your PR, or create an explanatory one for discussion. In many cases features are absent for a reason.
• This message body should clearly illustrate what problems it solves. If there are related issues, remember to reference them.
   WARN  Issues with latest peer dependency postcss-load-config@6 found  #634
• Ideally, include a test that fails without this PR but passes with it. PRs will only be merged once they pass CI. (Remember to run pnpm lint!)

Tests

• Run the tests with npm test or pnpm test

[dummdidumm]

Closing in favor of #640