[R] Enable object name linter

[asfimport]

We don't currently even try to use the object name lintr because identifying the S3 methods isn't quite right for Arrow in lintr. The basics of this is that we should be able to do something like head.ArrowClassName <- function() but if the head bit isn't correctly identified as an S3 method, lintr will complain thinking we are mixing dots and camel case. There are a number of different ways this doesn't fully work (see below). Once those are resolved, we should be able to enable an object name linter as well

A few PRs + issues around this:
r-lib/lintr#843
r-lib/lintr#841
r-lib/lintr#842
r-lib/lintr#846

Reporter: Jonathan Keane / @jonkeane

_{Note: This issue was originally created as ARROW-13537. Please see the migration documentation for further details.}