feat: get_file_size - gets the size of a file in bytes

[wassup05]

User facing function added is get_file_size(path [, err])

It returns the file size in bytes. It returns an error if the path is a directory or a symlink to a directory (because for the size of directories you will need to recurse into the directory and add all its component sizes to get its actual size)

It uses stat and _stati64 on POSIX and windows respectively. It returns an integer of kind int64 to handle large enough file sizes.

[perazz]

We have not used get_* patterns anywhere else so, this function may perhaps just be file_size? The fact that it is a function already means we're get_ting stuff.

[perazz]

This functionality can be achieved by Fortran intrinsics only. Perhaps that would be a good way to avoid cross-platform CI issues and avoid the C backend calls?

Suggested change

	type(state_type) :: err0
	integer :: code
	integer(c_int64_t) :: c_size
	if (is_directory(path)) then
	err0 = FS_ERROR("Is a directory!")
	call err0%handle(err)
	return
	end if
	code = stdlib_get_file_size(to_c_char(trim(path)), c_size)
	if (code /= 0) then
	err0 = FS_ERROR_CODE(code, c_get_strerror())
	call err0%handle(err)
	return
	end if
	type(state_type) :: err0
	integer :: code
	character(len=256) :: iomsg
	integer(c_int64_t) :: c_size
	if (is_directory(path)) then
	err0 = FS_ERROR("Is a directory!")
	call err0%handle(err)
	return
	end if
	inquire(file=path, size=c_size, iostat=code, iomsg=iomsg)
	if (code /= 0) then
	err0 = FS_ERROR_CODE(code, iomsg)
	call err0%handle(err)
	return
	end if