Fix indeterminate checkbox edge cases

[ryubro]

What I did

There were a few problems in the LionCheckboxIndeterminate:

1. When the element was connected, the last child was ignored because the _setOwnCheckedState() was called by form-element-register event before the same event was handled by the checkbox group which adds the element emitting the event to the formElements
2. When it had disabled child and all the other children were checked, clicking the indeterminate checkbox didn't respond, because the checked state was false although, in practice, it was fully checked
3. When the custom element's indeterminate property was true and the same property of the input node was also true, and clicking it should keep it indeterminated (like when one of the children is disabled and the others are mixed with checked and unchecked), the indeterminate property of the custom element doesn't change but the same property of the input node changes to false (seems like browser's default behavior) because no syncing could happen without the custome element's indeteminate property changing.

This PR fixes those issues.

[changeset-bot]

🦋 Changeset detected

Latest commit: 07467ef

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

This PR includes changesets to release 1 package

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@lion/ui	Patch

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[gerjanvangeest]

fixes: #1967

[okadurin]

Could you add a test with a use case that covers __removeFromSubCheckboxes?

[okadurin]

When there is one of children checkboxes disabled checked and the other child checkbox is unchecked, the parent should be unchecked. Now it is checked