Add Edits To Visible Range (#173)

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### Description

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This change adds all positive edits to the visible range. The assumption
is that any edits the user may make are going to be visible at some
point in time. If the cursor is not already on the page, it will be
scrolled to and if it extends beyond the viewport it will be scrolled to
be visible as well. In both of those cases the added range will be
overridden immediately due to the scroll. So this fixes the case where
characters are being inserted at the end of a file, but aren't being
scrolled to. This is apparent in the attached screen recordings.

Alternatives considered:
- **Re-calculating the visible set:** Decided against due to the extra
work that would need to be done potentially every keystroke to calculate
a new visible range.
- **Update the visible set only if the edit is actually visible:**
Decided against due to the reasons previously mentioned. Any edit will
eventually be included in the visible set if it is not already in it
when the edit occurs.

### Related Issues

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to one other -->

* closes #142
* Maybe related to #162

### Checklist

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- [x] I read and understood the [contributing
guide](https://github.com/CodeEditApp/CodeEdit/blob/main/CONTRIBUTING.md)
as well as the [code of
conduct](https://github.com/CodeEditApp/CodeEdit/blob/main/CODE_OF_CONDUCT.md)
- [x] The issues this PR addresses are related to each other
- [x] My changes generate no new warnings
- [x] My code builds and runs on my machine
- [x] My changes are all related to the related issue above
- [x] I documented my code

### Screenshots

Before fix. Edits applied to the end of the file are never sent to
tree-sitter for highlighting as they are out of the range of the visible
set. They are only updated when the document is scrolled and
`visibleSet` is recalculated.


https://user-images.githubusercontent.com/35942988/229874599-e2714b72-d51f-4748-89e1-b80ee51bddda.mov

After fix. Edits with positive deltas are always included in the
`visibleSet`.


https://user-images.githubusercontent.com/35942988/229875515-36e96921-71c0-48aa-a62c-75cbd91c52fb.mov


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Author: thecoolwinter

Sources/CodeEditTextView/Highlighting/Highlighter.swift

@@ -278,6 +278,9 @@ extension Highlighter: NSTextStorageDelegate {
         }
 
         let range = NSRange(location: editedRange.location, length: editedRange.length - delta)
+        if delta > 0 {
+            visibleSet.insert(range: editedRange)
+        }
 
         highlightProvider?.applyEdit(textView: self.textView,
                                      range: range,