Return ICollection instead of IList

[vanillajonathan]

Follow Microsoft's Framework Design Guidelines.

✔️ DO use Collection<T> or a subclass of Collection<T> for properties or return values representing read/write collections.

https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/guidelines-for-collections

[codecov]

Codecov Report

Merging #361 (84d6770) into master (7297a73) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #361   +/-   ##
=======================================
  Coverage   94.32%   94.32%           
=======================================
  Files           4        4           
  Lines         811      811           
  Branches       85       85           
=======================================
  Hits          765      765           
  Misses         34       34           
  Partials       12       12           

Impacted Files	Coverage Δ
src/HtmlSanitizer/EventArgs.cs	100.00% <100.00%> (ø)

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