macos: cleanup config margin lookup

this just slims down the number of lines drastically and makes it much
easier to review and compare things. i don't have a mac with which to
experiment and so i don't know whether any of the numbers could be tweaked
to simplify things further.

i find it odd that the default 20/23 is so much larger than the values
otherwise used. i also notice that several types, including float, are
missing here.

note that `CONFIG_ITEM_STRING_LIST` and `CONFIG_ITEM_RANGED_INTEGER` are
defined only within this file, seemingly just as a hack to get different
margins for such string/int variants.