globstar
If set, the pattern ** used in a filename expansion context will match all files and zero or more directories and subdirectories. If the pattern is followed by a ‘/’, only directories and subdirectories match.
-- The Shopt Builtin https://www.gnu.org/software/bash/manual/html_node/The-Shopt-Builtin.html

There is one thing we should decide before implementing it; Either

• globstar: Bool should be the second argument of file_glob() and file_glob_all() or
• new functions file_globstar() and file_globstar_all() should be added?

And furthermore, globstar should be enabled by default in amber?