Add String.indexOf(int ch, int fromIndex, int toIndex)

`String` currently exposes `indexOf(int ch)` and `indexOf(int ch, int fromIndex)`. In both cases, the upper limit for the search is implicitly the length of the target string. Sometimes this is too much, in particular when the character `ch` does not occur in the target `String`.

The implementations end up invoking internal methods supporting an explicit `int` argument `toIndex` to limit the search to positions with indices in the half-open range [`fromIndex`, `toIndex`). These internal methods are intrinsifed, and make use of SIMD instruction, where available, to considerably speed up the search.

Class `String` does not expose a method `indexOf(int ch, int fromIndex, int toIndex)`, with an explicit upper limit for the search. Since the underlying implementations already exist, it would be beneficial to add this method to `String`.