Add displaystyle as known property on math elements

[lounsbrough]

The math element supports a property called displaystyle which is not currently recognized by the no-unknown-property rule.

https://developer.mozilla.org/en-US/docs/Web/MathML/Global_attributes/displaystyle

[codecov]

Codecov Report

Merging #3652 (ea148fa) into master (eb5bb85) will not change coverage.
The diff coverage is n/a.

❗ Current head ea148fa differs from pull request most recent head 9ff9ef9. Consider uploading reports for the commit 9ff9ef9 to get more accurate results

@@           Coverage Diff           @@
##           master    #3652   +/-   ##
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  Coverage   97.65%   97.65%           
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  Files         132      132           
  Lines        9378     9378           
  Branches     3433     3433           
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  Hits         9158     9158           
  Misses        220      220           

Files	Coverage Δ
lib/rules/no-unknown-property.js	98.93% <ø> (ø)

[ljharb]

Thanks!