marker#deleteValueAtOffset detects if the character is outside the BMP

[bantic]

• acceptance tests for forward- and backward-deleting emoji
• audit the code for other places that may be affected

There remain some potential places where markers can still be affected by multi-code-point characters. Anywhere in models/marker.js that uses the native string-mutation methods (substr, e.g.) on this.value are affected, but deleteValueAtOffset is the only one that will typically trip on this. The other methods will normally be dealing with selections read from the DOM and browsers should not allow a user to select 1/2 of an emoji.

A more comprehensive fix is to write a set of UCS-2 safe string mutation functions and replace every use of built-in string mutation functions in models/marker.js with those.

fixes #274

[mixonic]

👏 👏 👏 👏 👏