Add support for \bigl, \bigr and friends

[Orace]

Those are described in the texbook p146.
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, the size are 1.2, 1.8, 2.4, 3.0 em