Fix the truncation of unicode strings

[davidchin]

What?

• Added a handlebars helper to truncate a string #106 added a truncation helper. It uses String.prototype.substring to truncate strings. However, the native function doesn't handle unicode characters properly. So, if you call the truncate helper on strings containing unicode characters (some non-English characters or emoji), you won't get what you expect.
• To save time, I decided to use a third party library called stringz. It has MIT license, so we should be good to use.

Why?

• This is necessary because we need to support multiple languages in Stencil.

Testing / Proof

• Unit

@bigcommerce/checkout @bigcommerce/stencil-team

[mcampa]

Awesome 👍