feat(GlLineChart): Defines a #tooltip-value slot

This change adds a new `#tooltip-value` which can be used to format
individual values in the tooltip of a line chart.

Usage:

Given a `format()` function, the tooltip can be formatted like this:

```vue
<gl-line-chart>
  <template #tooltip-value="{value}">{{format(value)}}</template>
</gl-line-chart>
```