fix: use searchParams in handler.js

[koheing]

Hi,
Thank you for a great library!

Summary

I change code in src/files/handler.js: use not query but searchParams.

Why

query is undefined in URL, so page.query property in load function always null.

routes/foo.svelte

<script lang="ts" context="module">
  import type { Load } from '@sveltejs/kit'
  export async function load({ page, fetch }: Parameters<Load>[0]) {
     const bar = page.query.get('xxx')  // If we access `http://***/?bar=123`, bar always null
  }
  ...
</script>

[jthegedus]

LGTM, thanks.

[github-actions]

🎉 This PR is included in version 0.5.2 🎉

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