Make ruler look more like Xcode. (#163)

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### Description

1. Add 20 pixels of inset to the left of the ruler and 8 pixels of inset
to the right of the ruler. I tried to match it as close as possible to
Xcode.
2. Set the background color of the highlight ruler line to the same
color as the highlighted line in the textView.

### Related Issues

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<!--- If this PR resolves the issue please specify (e.g. * closes #123)
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to one other -->

* closes #156
* closes #128

### 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

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<img width="765" alt="Screenshot 2023-03-16 at 19 05 59"
src="https://user-images.githubusercontent.com/82230675/225717006-40530a7b-7864-404b-8d14-5307f6f83bdb.png">


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Sources/CodeEditTextView/Controller/STTextViewController.swift

@@ -131,9 +131,12 @@ public class STTextViewController: NSViewController, STTextViewDelegate, ThemeAt
         rulerView.drawSeparator = false
         rulerView.baselineOffset = baselineOffset
         rulerView.font = NSFont.monospacedDigitSystemFont(ofSize: 9.5, weight: .regular)
+        rulerView.selectedLineHighlightColor = theme.lineHighlight
+        rulerView.rulerInsets = STRulerInsets(leading: 20, trailing: 8)
 
         if self.isEditable == false {
             rulerView.selectedLineTextColor = nil
+            rulerView.selectedLineHighlightColor = theme.background
         }
 
         scrollView.verticalRulerView = rulerView
@@ -249,6 +252,7 @@ public class STTextViewController: NSViewController, STTextViewDelegate, ThemeAt
 
         rulerView?.backgroundColor = useThemeBackground ? theme.background : .clear
         rulerView?.separatorColor = theme.invisibles
+        rulerView?.selectedLineHighlightColor = theme.lineHighlight
         rulerView?.baselineOffset = baselineOffset
         rulerView.highlightSelectedLine = isEditable