_.sortedLastIndex

Underscore has `_.sortedIndex` but this could be expanded to `_.sortedLastIndex` using a shared helper between them both (minimal code increase). This would allow `_.lastIndexOf` to support binary searches as well.

Start by locating the existing implementations and tests for _.sortedIndex and _.lastIndexOf in the Underscore repository. Read how sorted searches currently work and determine where a shared helper would fit. Done means _.sortedLastIndex is available and _.lastIndexOf can use binary-search behavior without regressing existing tests.

Written by the indexing model from the issue text.