Retain cursor position after reformatting (#1399)

* Restore cursor position based on old coordinates.
* Add source code for jsdiff and include it on the page
* Compute new line position after formatting based on text diffing.
* Compute new character position after formatting based on text diffing.
* Use diffChars() to recompute both line and column.
* Restore scroll position.
* Slight refactor.
* Use getDoc() to extract document instance.
* Compute cursor coordinates relative to the top-left corner of the editable document.

Author: Powell-v2

.gitignore

@@ -25,4 +25,5 @@ dist-*
 *.swp
 .stack-work
 stack.yaml.lock
-cabal.project.local
+cabal.project.local
+.DS_Store

third_party/jsdiff/LICENSE

@@ -0,0 +1,31 @@
+Software License Agreement (BSD License)
+
+Copyright (c) 2009-2015, Kevin Decker <[email protected]>
+
+All rights reserved.
+
+Redistribution and use of this software in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above
+  copyright notice, this list of conditions and the
+  following disclaimer.
+
+* Redistributions in binary form must reproduce the above
+  copyright notice, this list of conditions and the
+  following disclaimer in the documentation and/or other
+  materials provided with the distribution.
+
+* Neither the name of Kevin Decker nor the names of its
+  contributors may be used to endorse or promote products
+  derived from this software without specific prior
+  written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
+IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

third_party/jsdiff/diff.min.js

@@ -245,5 +245,6 @@
       type="text/javascript"
       src="https://cdn.jsdelivr.net/npm/[email protected]/dist/promise.min.js"
     ></script>
+    <script async src="js/diff.min.js"></script>
   </body>
 </html>