expand the handling of plurals by the `%` syntax

[StefanoMezzini]

Currently, {chk} messages and errors cannot handle the plural for "class" correctly. It can handle "directory" properly:

> message_chk("There %r %n problem director%y%s.", n = 1)
[1] "There is 1 problem directory."
> message_chk("There %r %n problem director%y%s.", n = 2)
[1] "There are 2 problem directories."

to match this behaviour, we would have to allow the %s%ses or %%es syntax to work so that we can switch between "class" and "classes".

> message_chk("There %r %n problem clas%s%es.", n = 1)
[1] "There is 1 problem clas%es."
> message_chk("There %r %n problem clas%s%es.", n = 2)
[1] "There are 2 problem class%es."
> message_chk("There %r %n problem class%%es.", n = 1)
[1] "There is 1 problem class%%es."
> message_chk("There %r %n problem class%%es.", n = 2)
[1] "There are 2 problem class%%es."

Alternatively we could add something like what {cli} does with "director{?y/ies}" to add irregular plurals (e.g., "clas%{s/ses}") and avoid breaking existing functionality.

[StefanoMezzini]

if we add {cli} as a dependency, we could handle pluralization quite easily and flexibly:

n <- 1
cli::pluralize("{n} class{?es}") # 1 class
cli::pluralize("{n} is {?= 1/=0 or > 1}") # 1 is = 1
n <- 0
cli::pluralize("{n} class{?es}") # 0 classes
cli::pluralize("{n} is {?= 1/=0 or > 1}") # 0 is =0 or > 1
n <- 2
cli::pluralize("{n} class{?es}") # 2 classes
cli::pluralize("{n} is {?= 1/=0 or > 1}") # 2 is =0 or > 1