fix(FormCheck): fix Form.Check.Label spacing (react-bootstrap#5983)

* Fix `Form.Check.Label` spacing

Currently, when using a `Form.Check.Label` component to customize
`Form.Check` rendering, there will be no space between the checkbox and
the label. This is because `Form.Check` is currently relying on the
presence of a `label` prop to apply the `form-check` class name to the
wrapping `<div>`, because checkboxes without labels [don't need the
wrapping element to have the `form-check` class name][1].

This commit adds a utility to check whether an element has a child of a
certain type. It then uses that utility to check if a `Form.Check`
element has a `Form.Check.Label` child and takes that into account when
determining whether the checkbox has a label.

Adding a special property (currently called `typeName`, but that can
certainly change) to components for this utility is necessary because
React minifies the `displayName` property in production.

[1]: react-bootstrap#5938 (comment)

* Add error handling to `includesType`

* Support mixing auto and custom child components

Currently, `Form.Check` doesn't allow users to use a custom
`Form.Check.Input` with a label generated by the `label` prop (and
requires that a custom `Form.Check.Input` be used if a custom
`Form.Check.Label` is also used) because the presence of _any_ children
in `Form.Check` prevents automatic inputs and labels (and feedback) from
generating. This approach allows mixing and matching by extracting any
custom input, label, and feedback components from `Form.Check`'s
`children` prop, then separately for each, using the custom component if
provided or an automatic component otherwise.

* Simply check `child.type`

The `type` property of React elements have reference equality with
matching imported component variables (`JSXElementConstructor` objects,
to be precise)! For some reason I was not expecting that, but it makes
sense to me now, since these constructor objects are being imported from
a singular source, so there's no reason React would create multiple
instances of them. So `child.type === type` all we need to check in
`getChildOfType`; the `typeName` prop I added earlier is not at all
needed.

* Add scripts for testing production build of docs

Just as a convenience. I can revert this commit if these scripts aren't
desired.

* Just check immediate children for `FormCheckLabel`

* Use correct variable

* Revert empty `.husky/pre-commit` change

No idea how that got into 0754b49.

* Revert "Add scripts for testing production build of docs"

This reverts commit 5343b0a.

Author: TyMick

src/ElementChildren.tsx

@@ -35,4 +35,17 @@ function forEach<P = any>(
   });
 }
 
-export { map, forEach };
+/**
+ * Finds whether a component's `children` prop includes a React element of the
+ * specified type.
+ */
+function hasChildOfType<P = any>(
+  children: React.ReactNode,
+  type: string | React.JSXElementConstructor<P>,
+): boolean {
+  return React.Children.toArray(children).some(
+    (child) => React.isValidElement(child) && child.type === type,
+  );
+}
+
+export { map, forEach, hasChildOfType };

src/FormCheck.tsx

@@ -8,6 +8,7 @@ import FormCheckLabel from './FormCheckLabel';
 import FormContext from './FormContext';
 import { useBootstrapPrefix } from './ThemeProvider';
 import { BsPrefixProps, BsPrefixRefForwardingComponent } from './helpers';
+import { hasChildOfType } from './ElementChildren';
 
 export type FormCheckType = 'checkbox' | 'radio' | 'switch';
 
@@ -152,7 +153,9 @@ const FormCheck: BsPrefixRefForwardingComponent<'input', FormCheckProps> =
         [controlId, id],
       );
 
-      const hasLabel = label != null && label !== false && !children;
+      const hasLabel =
+        (!children && label != null && label !== false) ||
+        hasChildOfType(children, FormCheckLabel);
 
       const input = (
         <FormCheckInput
@@ -172,7 +175,7 @@ const FormCheck: BsPrefixRefForwardingComponent<'input', FormCheckProps> =
             style={style}
             className={classNames(
               className,
-              label && bsPrefix,
+              hasLabel && bsPrefix,
               inline && `${bsPrefix}-inline`,
               type === 'switch' && bsSwitchPrefix,
             )}