String indexing

[annevk]

At a minimum we should clearly state that strings are 0-based and that you can index into them to obtain individual code points or code units.

This should also make the work in #110 more grounded as well as several existing Infra algorithms that operate on strings.

whatwg/html#7991 has some ideas for potentially doing more in terms of syntax, but it's not clear if that ends up paying for itself. Mainly mentioning that for completeness-sake.