Add extension methods to limit string size

[yringler]

I have multiple cases where I want to use a string, but want to keep it within a certain range.
For example, the key in a nosql DB, or a value in firebase analytics.
Thefor, I find it convenient to have an extension which can limit to max size, either from start or end.
I'll send a PR in a few minutes.

BTW, thanks for putting this together! It's great to have a central place with useful extension methods.