Scroll to visible on content view

The `scrollRangeToVisible` method now scrolls the content view to the
visible range, adjusting the target rect to account for the gutter view
width. This ensures the proper content area is made visible.

Also fix a small typo in the README.

Author: krzyzanowskim

README.md

@@ -116,7 +116,7 @@ The `STTextView` is an `NSView` subclass and can be initialized like any other v
 import STTextView
 
 let textView = STTextView()
-view.addSubView(textView)
+view.addSubview(textView)
 ```
 
 (macOS) add to scroll view

Sources/STTextViewAppKit/STTextView.swift

@@ -1267,7 +1267,11 @@ import AVFoundation
             rect = rect.inset(by: .init(top: 0, left: -textContainer.lineFragmentPadding, bottom: 0, right: -textContainer.lineFragmentPadding))
         }
 
-        return scrollToVisible(rect)
+        // scroll to visible IN clip view (ignoring gutter view overlay)
+        // adjust rect to mimick it's size to include gutter overlay
+        rect.origin.x -= gutterView?.frame.width ?? 0
+        rect.size.width += gutterView?.frame.width ?? 0
+        return contentView.scrollToVisible(rect)
     }
 
     open func scrollRangeToVisible(_ range: NSRange) {