feat(web): Custom layer targets for Shift double-tap

Applies proposed feature https://community.software.sil.org/t/testing-caps-on-touch/6849/5:

1. A `caps` layer must still exist in the keyboard.
2. If there is at least one multitap subkey defined on the Shift key on
   the active layer, then the `nextlayer` property of the first multitap
   subkey for the Shift key will be used instead of the default `caps`.
3. This does not change the behavior of the Shift key otherwise.

For example, the following defines a valid multitap key on Shift on the
`rightalt` layer to switch to a custom extended `rightalt-shift-caps`
layer.

This does not implement any other multitap functionality. It just allows
the keyboard developer to specify a target layer for a given double-tap.

```
  {
    "id": "K_SHIFT",
    "text": "*Shift*",
    "width": 150,
    "sp": 1,
    "nextlayer": "rightalt-shift",
    "layer": "default",
    "multitap": [
      {
        "text": "",
        "id": "T_new_694",
        "sp": "1",
        "nextlayer": "rightalt-shift-caps"
      }
    ]
  },
```