Special treatment for filesize 0b (#133)

This PR is a partial fix for file size `0b` which will lead to parse
error in current grammar.

<img width="388" alt="image"
src="https://github.com/user-attachments/assets/81d231fd-b1c8-4d19-a2db-0ea345d4555b">

Since `0b` is a special single terminal token in `$.val_number` and
`$.val_binary`.
A perfect split into `value` 0 and `unit` b needs unnecessarily
complicated tweaking of potential conflicts (also need to split `0b` in
other 2 rules).

With this PR however, `0b` is added to `$.val_filesize` as a special
case, no detailed fields generated:

```scheme
  (pipeline [0, 0] - [1, 0]
    (pipe_element [0, 0] - [0, 2]
      (val_filesize [0, 0] - [0, 2])))
```

But it avoids the error with minimal effort.