Make the .roles parameter of set_main_contributors() work with a combination of named and unnamed expressions

[arnaudgallou]

Currently, the only ways to set the same main contributors across all but a few roles is to specify main contributors for each roles, or to call set_main_contributors() twice, once to set main contributors across all roles and another time to define main contributors more specifically. E.g.:

aut <- Plume$new(tibble::tibble(
  given_name = LETTERS[1:3],
  family_name = LETTERS[1:3],
  analysis = 1,
  writing = 1,
  methodology = 1,
))
aut$set_main_contributors(3, .roles = aut$get_roles())
aut$set_main_contributors(writing = 2)
aut$get_contributions()
#> Formal analysis: C.C., A.A. and B.B.
#> Methodology: C.C., A.A. and B.B.
#> Writing - original draft: B.B., A.A. and C.C.

I could combine these features so that .roles only applies to unnamed expressions while named expressions would work as normal:

# set the third author as main contributor to all roles but writing
aut$set_main_contributors(3, writing = 2, .roles = aut$get_roles())