Add support for \bigl, \bigr and friends

Those are described in [_the texbook_ p146](https://archive.org/details/texbook00dona/page/146).
And are macro:
https://github.com/TeX-Live/texlive-source/blob/4f771e41a6c3799e9d16e44633c7fa95dc41f1bc/texk/web2c/tests/basic.tex#L897-L908

- [ ] `\bigl`, `\bigr`: _just enough bigger [...] so that the difference can be perceived_, need to be clarified
- [ ] `\Bigl`, `\Bigr`: 50% taller than `\big_` aka ×1.5
- [ ] `\biggl`, `\biggr`: twice the size of `\big_` aka ×2.0
- [ ] `\Biggl`, `\Biggr`: 2.5 times as tall as `\big_` aka ×2.5

P.S. Based on [this discussion](https://github.com/TeX-Live/yhmath/issues/3#issuecomment-467512722), the size are 1.2, 1.8, 2.4, 3.0 em