chore: Stop Extensions from importing FieldDropdown

[mikeharv]

The basics

• I validated my changes

The details

Proposed Changes

This moves checkDropdownOptionsInTable out of Extensions and into FieldDropdown. Extensions now includes a setter for this checker so that the field can provide its own. field_dropdown.ts now registers the checker once at module initialization time .Subclasses like FieldVariable, FieldGridDropdown, and FieldDependentDropdown should not need updating.
An error is thrown just in case an application somehow uses the extension without ever importing field_dropdown.ts or installing a custom checker.

Reason for Changes

The current dependency chain is problematic. As part of #1276, I expect to need to be able to value import {BlockSvg}. The parent Block imports Extensions, which means there's a potential for circular dependency. Given that this checker is strictly used for dropdown fields, moving it out feels like a reasonable way around this issue.

Test Coverage

Manually confirmed that tooltips are still shown and nothing is thrown. All tests continue to pass.