infix_spaces_linter should include tilde / formula operator and = in function definitions

[MichaelChirico]

It's covered by {styler}:

styler::style_text("a~b")
# a ~ b

But not infix_spaces_linter():

lintr::lint("a~b\n", infix_spaces_linter())

This is called out by omission in the style guide, which talks about one-sided formulas:

https://style.tidyverse.org/syntax.html#infix-operators

[MichaelChirico]

lintr also misses = used in function definitions:

lintr::lint("foo <- function(a=1, b=2) { }\n", lintr::infix_spaces_linter())

[MichaelChirico]

This was handled in #931