Add SchemaPresetsAndDefault check

[web-orbit12]

What are you adding in this PR?

Fixes #750

A section schema that defines both presets and default passes Theme Check today, but shopify theme dev rejects it with:

Invalid schema: cannot define both 'default' and 'presets'

This adds a SchemaPresetsAndDefault check that reports the same error in the editor and in shopify theme check, highlighting the default value.

• Runs on section files only: theme block schemas have no default property.
• Only reports on otherwise valid schemas, like the other SchemaPresets* checks.
• The message matches the platform error word for word.

What's next? Any followup issues?

The check needs a page on shopify.dev (checks/schema-presets-and-default), which a Shopify team member would have to add.

I treated key presence as the trigger, so an empty presets: [] with default is also flagged. Happy to switch to non-empty presets only if the platform accepts that combination.

Tophatting

• Put the schema below in sections/test.liquid: the error appears on default. Remove either key and it goes away.

{% schema %}
{
  "name": "Test",
  "presets": [{ "name": "Test" }],
  "default": { "settings": {} }
}
{% endschema %}

• Tested in the playground (pnpm playground) with the section above, built from main and from this branch:

Before (main @ 8bc5e03): no error

After (this branch @ d4dcdf9): default is flagged

• pnpm vitest run packages/theme-check-common/src/checks/schema-presets-and-default (5 tests), plus the full theme-check-common and theme-check-node suites pass.

• I added screenshots of the changes (before and after the changes if applicable)

Before you deploy

• This PR includes a new checks or changes the configuration of a check
  • I included a minor bump changeset
  • I ran pnpm build and committed the updated configuration files
    • If applicable, I've updated the theme-app-extension.yml config (not applicable: in app mode, sections have no schema to check, so the check is a no-op)

[web-orbit12]

I have signed the CLA!