Add a `IndecesOf()` Extension Method for `string`

[JasonBock]

Basically, I want to look for all the indexes of a given string (and char) within a string. For a string, may need an overload to look for overlaps. Like, if the string was this:

var value = "abcabcabc";

If I did this:

var indexes = value.IndecesOf("abcabc");

Should that return [0, 3], or just [0]? Maybe provide an optional argument, like an IndexSearch enum with Unique or Overlap.

Would also need overloads for all of the culture/formatting values.