FileAccess: Introduce advance() for skipping bytes

[cruglet]

This adds a advance(offset) method to FileAccess, providing a more readable & streamlined way to move the file cursor forward/backward. Unless I'm missing something, the only way to do this is by manually calling seek(file.get_position() + amount), which feels unnecessarily verbose for an operation like this. While you can also use methods like get_32() or get_buffer() to advance the cursor, these actually read data into memory, which is something that can negatively impact performance when skipping over large segments (it's also not very explicit that these functions do that). This method offers a more readable and purpose-specific alternative.