Copy operations include an "ln" field that is used to update the line
number of the lines being copied. If we ignore it, we end up with
inconsistent line numbers in the cache.

For instance, consider the following file (`|` indicates the cursor
position):

```
a|
b
```

On pressing "Enter" to insert a newline, the following update is being
sent:

{"update":{"annotations":[{"n":1,"payloads":null,"ranges":[[1,0,1,0]],"type":"selection"}],"ops":[{"lines":[{"ln":1,"styles":[0,2,8],"text":"a\n"},{"cursor":[0],"ln":2,"styles":[0,1,8],"text":"\n"}],"n":2,"op":"ins"},{"n":1,"op":"skip"},{"ln":3,"n":1,"op":"copy"}],"pristine":false},"view_id":"view-id-2"}

The intersting operation is {"ln":3,"n":1,"op":"copy"}.
Before the update we have the following line cache:

LineCache { invalid_before: 0, lines: [Line { text: "a", cursor: [1], styles: [StyleDef { offset: 0, length: 2, style_id: 8 }], line_num: Some(1) }, Line { text: "b", cursor: [], styles: [StyleDef { offset: 0, length: 1, style_id: 8 }], line_num: Some(2) }], invalid_after: 0 }

If we ignore the "ln" field, we end up with the following cache where
we have to lines number "2" ("" and "b"):

LineCache { invalid_before: 0, lines: [Line { text: "a", cursor: [], styles: [StyleDef { offset: 0, length: 2, style_id: 8 }], line_num: Some(1) }, Line { text: "", cursor: [0], styles: [StyleDef { offset: 0, length: 1, style_id: 8 }], line_num: Some(2) }, Line { text: "b", cursor: [], styles: [StyleDef { offset: 0, length: 1, style_id: 8 }], line_num: Some(2) }], invalid_after: 0 }

because the "b" line is copied as is. With this commit, we end up with
the following line cache:

LineCache { invalid_before: 0, lines: [Line { text: "a", cursor: [], styles: [StyleDef { offset: 0, length: 2, style_id: 8 }], line_num: Some(1) }, Line { text: "", cursor: [0], styles: [StyleDef { offset: 0, length: 1, style_id: 8 }], line_num: Some(2) }, Line { text: "b", cursor: [], styles: [StyleDef { offset: 0, length: 1, style_id: 8 }], line_num: Some(3) }], invalid_after: 0 }