Fix multi line paste detection and filtering (microsoft#8634)

- Detect `\r` when warning about multi line paste
- Translate `\n` to `\r` on paste

## PR Checklist
* [x] Closes microsoft#8601
* [x] Closes microsoft#5821

## Validation Steps Performed
Manual testing

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -1935,8 +1935,9 @@ namespace winrt::TerminalApp::implementation
                 }
             }
 
-            const bool hasNewLine = std::find(text.cbegin(), text.cend(), L'\n') != text.cend();
-            const bool warnMultiLine = hasNewLine && _settings.GlobalSettings().WarnAboutMultiLinePaste();
+            const auto isNewLineLambda = [](auto c) { return c == L'\n' || c == L'\r'; };
+            const auto hasNewLine = std::find_if(text.cbegin(), text.cend(), isNewLineLambda) != text.cend();
+            const auto warnMultiLine = hasNewLine && _settings.GlobalSettings().WarnAboutMultiLinePaste();
 
             constexpr const std::size_t minimumSizeForWarning = 1024 * 5; // 5 KiB
             const bool warnLargeText = text.size() > minimumSizeForWarning &&

src/cascadia/TerminalControl/TermControl.cpp

@@ -2025,6 +2025,7 @@ namespace winrt::Microsoft::Terminal::TerminalControl::implementation
     // - Pre-process text pasted (presumably from the clipboard)
     //   before sending it over the terminal's connection, converting
     //   Windows-space \r\n line-endings to \r line-endings
+    // - Also converts \n line-endings to \r line-endings
     void TermControl::_SendPastedTextToConnection(const std::wstring& wstr)
     {
         // Some notes on this implementation:
@@ -2034,20 +2035,51 @@ namespace winrt::Microsoft::Terminal::TerminalControl::implementation
         //   performance guarantees aren't exactly stellar)
         // - The STL doesn't have a simple string search/replace method.
         //   This fact is lamentable.
-        // - This line-ending conversion is intentionally fairly
-        //   conservative, to avoid stripping out lone \n characters
-        //   where they could conceivably be intentional.
+        // - We search for \n, and when we find it we copy the string up to
+        //   the \n (but not including it). Then, we check the if the
+        //   previous character is \r, if its not, then we had a lone \n
+        //   and so we append our own \r
 
-        std::wstring stripped{ wstr };
+        std::wstring stripped;
+        stripped.reserve(wstr.length());
 
         std::wstring::size_type pos = 0;
+        std::wstring::size_type begin = 0;
 
-        while ((pos = stripped.find(L"\r\n", pos)) != std::wstring::npos)
+        while ((pos = wstr.find(L"\n", pos)) != std::wstring::npos)
         {
-            stripped.replace(pos, 2, L"\r");
+            // copy up to but not including the \n
+            stripped.append(wstr.cbegin() + begin, wstr.cbegin() + pos);
+            if (!(pos > 0 && (wstr.at(pos - 1) == L'\r')))
+            {
+                // there was no \r before the \n we did not copy,
+                // so append our own \r (this effectively replaces the \n
+                // with a \r)
+                stripped.push_back(L'\r');
+            }
+            ++pos;
+            begin = pos;
+        }
+
+        // If we entered the while loop even once, begin would be non-zero
+        // (because we set begin = pos right after incrementing pos)
+        // So, if begin is still zero at this point it means we never found a newline
+        // and we can just write the original string
+        if (begin == 0)
+        {
+            _connection.WriteInput(wstr);
+        }
+        else
+        {
+            // copy over the part after the last \n
+            stripped.append(wstr.cbegin() + begin, wstr.cend());
+
+            // we may have removed some characters, so we may not need as much space
+            // as we reserved earlier
+            stripped.shrink_to_fit();
+            _connection.WriteInput(stripped);
         }
 
-        _connection.WriteInput(stripped);
         _terminal->TrySnapOnInput();
     }