Extend assignment_linter() to enforce '=' by default for assignment

[MichaelChirico]

Currently, we don't have a way for enforcing a style guide rule other than <-, but not all authors prefer this.

We should extend assignment_linter() to allow for a different default operator (with either <- or = allowed).

[IndrajeetPatil]

Agreed. This will also help users who prefer (e.g.) {formatR} over {styler} as a formatter for R:

library(formatR)
tidy_source(text = "x = 1")
#> x = 1

^{Created on 2023-12-14 with reprex v2.0.2}