Subsume assignment_spaces_linter as part of infix_spaces_linter

[MichaelChirico]

Follow-up from

https://github.com/AshesITR/lintr/pull/1#discussion_r820137602

is there any reason to treat <- differently than other infix operators?

is it sufficient to just toggle which operators to exclude with an argument to the linter?

[MichaelChirico]

infix_spaces_linter already triggers a lint:

lintr::lint("a<-1\n", lintr::infix_spaces_linter())
# <text>:1:2: style: Put spaces around all infix operators.
# a<-1
# ~^~~

[MichaelChirico]

Turns out assignment_spaces_linter was added in this release 😅 #538