Sets the [property@SnippetChunk:focus-position] property.
The focus-position is used to determine how many tabs it takes for the snippet to advanced to this chunk.
A focus-position of zero will be the last focus position of the snippet and snippet editing ends when it has been reached.
A focus-position of -1 means the chunk cannot be focused by the user.
| this | |
| focus_position |
the focus-position |