feat: warn the user when they forget to make boolean inputs optional in their form schemas

[ottomated]

closes #15785
closes #15786

Through typescript trickery, prevent form schemas from containing non-optional booleans, i.e.:

// Not allowed:
form(v.object({
  checkbox: v.boolean(),
}));

// Allowed:
form(v.object({
  checkbox: v.optional(v.boolean(), false)
}));

This prevents the pitfall of forgetting that <input type="checkbox"> elements don't send a false value when they're unchecked.

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