Make posFromIndex and indexFromPos take lineSeparator into account

Closes codemirror#3905

Author: marijnh

lib/codemirror.js

@@ -7646,9 +7646,9 @@
     },
 
     posFromIndex: function(off) {
-      var ch, lineNo = this.first;
+      var ch, lineNo = this.first, sepSize = this.lineSeparator().length;
       this.iter(function(line) {
-        var sz = line.text.length + 1;
+        var sz = line.text.length + sepSize;
         if (sz > off) { ch = off; return true; }
         off -= sz;
         ++lineNo;
@@ -7659,8 +7659,9 @@
       coords = clipPos(this, coords);
       var index = coords.ch;
       if (coords.line < this.first || coords.ch < 0) return 0;
+      var sepSize = this.lineSeparator().length;
       this.iter(this.first, coords.line, function (line) {
-        index += line.text.length + 1;
+        index += line.text.length + sepSize;
       });
       return index;
     },