RegularExpression: A few small features for CMake

This MR adds a few small features for implementing cmake/cmake#26629 (closed).

• An options parameter to RegularExpression::find() with two options.
• A new method num_groups() that counts capture subgroups.
• Less overloads and more null checks for RegularExpressionMatch::start/end/match mehtods.

Only one of those features (the BOL_AT_OFFSET option) is needed for cmake/cmake!10221 (merged). I can make it a separate MR if you wish.

mentioned in merge request cmake/cmake!10221 (merged)

added 1 commit

• c8f5f4bf - RegularExpression: Add an options parameter to find()

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Do: stage

assigned to @brad.king

resolved all threads

Do: merge

mentioned in commit 511adf49

merged manually

mentioned in merge request cmake/cmake!10247 (merged)