String Additions

String capitalization. Index-based insertion, replacement, and removal. Similar implementation of C# String.Format(..) to allow for terse index-based string interpolation without needing to use a literal with '$var' at the time of interpolation.
add tests for:
* `overwrite < 0`: throws
* `overwrite == 0`: behaves as an insert
* `overwrite > original.length - startIndex`: replaces everything after startIndex