Add incremental directory iterator

[rovolsw]

Related PR:

• 88: Add nob_delete_dir #103

This PR features an incremental directory iterator to nob.h.

New functions in this PR:

• nob_is_dir_empty()
• nob_dir_iter_open()
• nob_dir_iter_next()
• nob_dir_iter_close()
• nob_dir_iter_getname()

New structure in this PR:

• Nob_Dir_Iter

Ues cases:

• Walking directory recusively
• Deleting directory recusively
• etc.

Example:

Nob_Dir_Iter iter = {0};
if (nob_dir_iter_open(&iter, "./src")) {
    while (nob_dir_iter_next(&iter)) {
        const char *name = nob_dir_iter_getname(iter);
        // processing
    }
    nob_dir_iter_close(iter);
}