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"
}
]
},
```