[form-builder] Show warning for array items without _type

[rexxars]

Type of change (check at least one)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or update to documentation)
• Maintenance
• Other, please describe:

Does this change require a documentation update? (Check one)

• Yes
• No

Current behavior

If an array member type has a name, but array items exist that do not have a _type, they are silently hidden.

Example: https://test-studio-git-next.sanity-io.now.sh/test/desk/arraysTest;untyped-objects

• "Array of multiple types" actually has one array item, but is not displayed

Description

This PR checks for the case when a member type cannot be found.

• If there is only a single member type that is valid, it renders an option to set the type name
• If there are multiple member types that are valid, it renders the possible types and an option to remove the item

Example: https://test-studio-git-fix-array-missing-type.sanity-io.now.sh/test/desk/arraysTest;untyped-objects

Note for release

• Fixed bug where objects in arrays without a _type property would not be displayed

Checklist

• I have read the Contributing Guidelines
• The PR title includes a link to the relevant issue
• The PR title is appropriately formatted: [some-package] PR title (#123)
• The code is linted
• The test suite is passing
• Corresponding changes to the documentation have been made

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[bjoerge]

Looks great, thank you!