[Fix] no-unknown-property: allow fill prop on <symbol>

[stefanprobst]

this adds fill to the list of allowed props for <symbol> in no-unknown-props. see https://developer.mozilla.org/en-US/docs/Web/SVG/Element/symbol#global_attributes

[codecov]

Codecov Report

Merging #3555 (1122738) into master (f56851b) will increase coverage by 97.61%.
The diff coverage is n/a.

❗ Current head 1122738 differs from pull request most recent head 492c34c. Consider uploading reports for the commit 492c34c to get more accurate results

@@             Coverage Diff             @@
##           master    #3555       +/-   ##
===========================================
+ Coverage        0   97.61%   +97.61%     
===========================================
  Files           0      132      +132     
  Lines           0     9289     +9289     
  Branches        0     3396     +3396     
===========================================
+ Hits            0     9067     +9067     
- Misses          0      222      +222     

Impacted Files	Coverage Δ
lib/rules/no-unknown-property.js	98.82% <ø> (ø)

... and 131 files with indirect coverage changes

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[ljharb]

Thanks!