PrettyPrinterPerformance Optimized the PrettyPrinter for #894

Worked to get the perfomance to be closer to where we were before the changes in #883. This code should be only about 1.5% slower rather than 7% slower.

Sources/SwiftFormat/PrettyPrint/PrettyPrintBuffer.swift

@@ -122,16 +122,15 @@ struct PrettyPrintBuffer {
     // In case of comments, we may get a multi-line string.
     // To account for that case, we need to correct the lineNumber count.
     // The new column is only the position within the last line.
-    let lines = text.split(separator: "\n")
-    lineNumber += lines.count - 1
-    if lines.count > 1 {
-      // in case we have inserted new lines, we need to reset the column
-      column = lines.last?.count ?? 0
-    } else {
-      // in case it is an end of line comment or a single line comment,
-      // we just add to the current column
-      column += lines.last?.count ?? 0
+    let lines = text.count { $0 == "\n" }
+    lineNumber += lines
+    guard lines > 1, let lastNewlineIndex = text.lastIndex(of: "\n") else {
+      // Handle case where only one line exists.
+      column += text.count
+      return
     }
+    let lastLine = text[text.index(after: lastNewlineIndex)...]
+    column = lastLine.count
   }
 
   /// Request that the given number of spaces be printed out before the next text token.