assignment_linter() gains operator= argument to allow, e.g. '=' assignments

[MichaelChirico]

Supersedes #2698 and #2521. h/t again @J-Moravec for getting the ball rolling here.

Closes #2441.

This also un-couples the linted operator from the allow_trailing behavior; lints generated by the operator used are now independent of lints generated by line-trailing assignment operators:

## current 'main'
lint("a <-
1", assignment_linter(allow_trailing=FALSE))
# <text>:1:3: style: [assignment_linter] Assignment <- should not be trailing at the end of a line.
# a <-
#   ^~
lint("a =
1", assignment_linter(allow_trailing=FALSE))
# <text>:1:3: style: [assignment_linter] Use <-, not =, for assignment.
# a =
#   ^

## current branch (assignment-operator)
lint("a <-
1", assignment_linter(allow_trailing=FALSE))
# <text>:1:3: style: [assignment_linter] Assignment <- should not be trailing at the end of a line.
# a <-
#   ^~
lint("a =
1", assignment_linter(allow_trailing=FALSE))
# <text>:1:3: style: [assignment_linter] Use one of <-, <<- for assignment, not =.
# a =
#   ^
# <text>:1:3: style: [assignment_linter] Assignment = should not be trailing at the end of a line.
# a =
#   ^

Having <<- allowed by default is back-compatible, but does strike me as odd -- should we also plan to remove this from the defaults?