Takes either a function or a relative length, just like with `lr`,
`stretch`, and `accent` which was changed in the previous two commits.
The default is now much clearer to the user: `x => x * 1.1 - 0.1em`.
Previously there was always a short fall when scaling delimiters, even if
the user requested a specific size. This is no longer the case; the short
fall is only present in the default for `lr` (`x => x - 0.1em`) - the
size of the delimiters is now actually what was specified in the size
argument. This also makes the default for `lr` much clearer to the user.
A slight hack was used by exploiting the `name` property in the `func`
attribute macro so that the default value in the docs for `lr.size` would
clearly show what the default function was (instead of just its name
`default_lr_size` which is meaningless and inaccessible to the user).
