feat(GridForm, ConnectedForm): Add nested checkboxes

This PR introduces comprehensive nested checkbox functionality to the `GridForm` and `ConnectedForm` components, allowing for hierarchical checkbox selections with proper indeterminate states and accessibility support.

Author: aresnik11

packages/gamut/__tests__/__snapshots__/gamut.test.ts.snap

@@ -21,6 +21,7 @@ exports[`Gamut Exported Keys 1`] = `
   "ConnectedForm",
   "ConnectedFormGroup",
   "ConnectedInput",
+  "ConnectedNestedCheckboxes",
   "ConnectedRadio",
   "ConnectedRadioGroup",
   "ConnectedRadioGroupInput",