rorynolan/filesstrings: `match_arg()` and `nth_number_after_mth()`

2.4.0 NEW FEATURES

• Added nth_number_after_mth() and friends (there are 8 friends).

• Defunct functions are now gone completely (as opposed to triggering a .Defunct() call).

• Add an overwrite argument to file.move() with default value FALSE to ensure that files are not accidentally overwritten.

• Speed up extract_numbers().

• Fix some bad typing in C++ code.
• remove_dirs() was claiming to have deleted directories that weren't there.

• There is a new function match_arg() for argument matching which is inspired by RSAGA::match.arg.ext(). Its behaviour is almost identical but RSAGA is a heavy package to depend upon so filesstrings::match_arg() might be handy for package developers.