@lute/fs: clean up some smaller problems in the filesystem implementation code

[aatxe]

This PR implements a bunch of small fixes/clean up work for various things across the implementation of @lute/fs, including:

• Fixes a typo in the name of initializeFS where it was mistakenly initalizeFS
• Rename some of the variables in setFlags to have clearer names without shadowing
• Change createDurationFromTimespec32 to just be an overload of createDurationFromTimespec without suggesting any particular bit sizing
• Remove the std::fill zero-pass over the read chunk buffer in FSRead::readCallback since only bytesRead bytes are ever appended to the output buffer, meaning nothing ever actually looks at the zeroed bytes.
• Change the behavior of append (a) mode to include O_CREAT to make it consistent with general POSIX behavior where append will create non-existent files.
• Eliminate some duplicated logic for the situation where the mode for files was not present.

The biggest/most disruptive change here is a change to how we validate arguments to functions. Looking at the Luau VM code and the existing built-in libraries for Luau, it's actually very abnormal to do lua_gettop and produce errors for argument counts and so forth. The general pattern is that functions will do typechecks for the arguments they're expecting, and silently ignore any additional arguments. I've changed the filesystem library to be consistent with this behavior in this PR, and I'll put up a follow-up PR to make this change across the whole project.