chore: add migration guide from eslint-plugin-svelte3

[baseballyama]

fix: #252

Honestly, I couldn't find difficulties in migrating from eslint-plugin-svelte3.
But preparing this section is essential for new users.
In the future, I will follow it up in the docs if some new user hits difficulties.

And I handled sveltejs/kit#6847 (comment) in the PR.

[changeset-bot]

⚠️ No Changeset found

Latest commit: c223953

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

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

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

[ota-meshi]

Thank you very much!