Add selection helper

NOTE: It is hidden by default styles. Can be enabled by adding this CSS

.json-inspector__key,
.json-inspector__value {
    position: relative;
}

.json-inspector__selection {
    position: absolute;
    top: -1px;
    left: -3px;
    right: 0;
    padding: 0 3px;

    outline: none;
    border: none;
    opacity: 0;
    line-height: 1;
    box-shadow: 3px 3px 4px rgba(0, 0, 0, 0.5), 0 0 0 1px rgba(0, 0, 0, 0.3);
    color: #222;
}

.json-inspector__selection:focus {
    opacity: 1;
}
3.0.5