added ALTREP to _selfrefok

[mattdowle]

Closes #4734

Not adding a test as the MRE requires dplyr. Adding dplyr as a Suggest would be too heavy a dependency chain. In this case, the fix is clear and straightforward. The nature of the fix in this case means we're ok without a test. I did try and trace through what's happening in dplyr::mutate, and it's maybe something to do with names going through as_utf8_character. When I saw lines like cols <- vec_recycle_common(!!!cols, .size = nrow(data)) with the three !!!, though, I'm lost.

I tested locally and it works now. Let's see if anything else comes up after this fix.

[codecov]

Codecov Report

Merging #4756 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4756   +/-   ##
=======================================
  Coverage   99.44%   99.44%           
=======================================
  Files          73       73           
  Lines       14539    14539           
=======================================
  Hits        14458    14458           
  Misses         81       81           

Impacted Files	Coverage Δ
src/assign.c	99.70% <100.00%> (ø)

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