RegularExpression: Add an options parameter to find()

There are two options:

 * BOL_AT_OFFSET enables matching the "^" anchor at the search
   start position instead of the input start. This is the legacy
   CMake behavior.

 * NONEMPTY_AT_OFFSET rejects empty matches at the search start
   position. This is used to allow zero-length matches when
   searching for multiple match occurrences.

Issue: cmake/cmake#26629