missing_argument_linter() misses S4 usage

[MichaelChirico]

Uncovered by #2191

lint(text = "obj$get(a =)", linters = missing_argument_linter())

The reason is that get is not a <SYMBOL_FUNCTION_CALL> in the AST -- it's a <SLOT>! (TIL)

We might need to consider more broadly which //SYMBOL_FUNCTION_CALL linters also apply when the call is a //SLOT.

We do so in one place so far:

lintr/R/keyword_quote_linter.R

Line 84 in c315baf

| //OP-AT/following-sibling::SLOT[starts-with(text(), '`')]