feat: improve `svelte/valid-compile` to use `svelte.config.js`'s `onwarn`

[ota-meshi]

close #311

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: e1e85f2ea67dd11e15f4c783a2834f852327aff1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | -------------------- | ----- | | eslint-plugin-svelte | Minor |

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR