This limits the levels of recursion allowed when encountering multiple
non-adjacent `**` portions of a pattern.

Update `**` handling, with performance massively improved by limiting
the recursive walk much more aggressively. When a `**` portion is
present, the entire pattern is split up into sections. The head and tail
first have to match, and then each subsequent portion is only tested in
the part of the file where it might actually be found, taking advantage
of the fact that non-globstar portions must always consume as many path
portions as there are pattern portions.

Fix: GHSA-7r86-cg39-jmmj

This flattens nested extglobs where that can be done correctly.

In other cases, the recursion into nested extglobs is limited to
2, in order to prevent runaway ReDOS patterns.

fix: GHSA-23c5-xmqv-rm74

Fix: #284