add quick shortcut emoji feature and tests

Signed-off-by: macekj <macekj@umich.edu>

src/editor/parts.ts

@@ -190,7 +190,9 @@ abstract class PlainBasePart extends BasePart {
                 return true;
             }
             // only split if the previous character is a space
-            return this._text[offset - 1] !== " ";
+            // or if it is a + and this is a :
+            return this._text[offset - 1] !== " " &&
+                (this._text[offset - 1] !== "+" || chr !== ":");
         }
         return true;
     }