Add `allow_multiple_spaces = TRUE` to `infix_spaces_linter()` to fully replace `assignment_linter()`

[AshesITR]

Since @f-ritter's original use-case was disallowing these, I think we should also support that rule via an argument (allow_multiple_spaces = TRUE). WDYT?

Originally posted by @AshesITR in #931 (comment)

NB: The implementation will likely be easier using XPath instead of ids_with_token.

[MichaelChirico]

Copying comment from the thread too:

should be a lot easier if we convert the linter to XPath at the same time (requires something like following-sibling::[@col1 = preceding-sibling::/@Col2] vs. > preceding-sibling::*/@Col2 iinm)

[MichaelChirico]

NB: be sure to offer attribution for this fix when this is filed. The following was lost in #931

New assignment_spaces() lintr. (#538, @f-ritter)