Add binary logarithm

[dltmtt]

This package already defines the missing trigonometric functions, so I thought we could add the missing logarithmic function too (as defined in ISO 80000-2:2019).
I added another row in the table in the manual since this is not a Typst built-in, so it should not have the built-in annotation.

[Leedehai]

Thanks! Though normally I'd like to have a high bar for new op symbols, but since ln and lg are already there, it doesn't hurt to add the little-used lb here for completeness.