LibFileSystem+Userland: Rename size() to size_from_stat()

This reflects what the functions does more accurately, and allows for
adding functions to get sizes through other methods.

This also corrects the return type of said function, as size_t may only
hold sizes up to 4GB on 32-bit platforms.

Userland/Libraries/LibFileSystem/FileSystem.cpp

@@ -350,7 +350,7 @@ ErrorOr<void> remove(StringView path, RecursionMode mode)
     return {};
 }
 
-ErrorOr<size_t> size(StringView path)
+ErrorOr<off_t> size_from_stat(StringView path)
 {
     auto st = TRY(Core::System::stat(path));
     return st.st_size;