Adds a `replacementSpan` field on a completion that will allow typed
text to be replaced with the suggested completion value if a user
selects the completion. Previously, the completion value would simply be
appended to the text already typed. E.g. if we had

```
{{ti}}
```

typed in a template and `title` was recommended as a completion and
selected, the template would become

```
{{tititle}}
```

With `replacementSpan`, the original text `ti` will be replaced for
`title`.