Support CSS declarations with multiple selectors

[swhitty]

Fixes #93

StyleSheet declarations with multiple Selectors are now useable:

.a, .b {
    fill: red;
}

Both class="a" and class="b" will now receive fill: red;

Additionally duplicate selectors correctly cascade with the last attribute winning:

.a {
   fill: red;
}
.a {
  stroke: blue;
}
.a {
   fill: purple;
}

Will resolve to

.a {
   fill: purple;
   stroke: blue;
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (1355dce) to head (3553cd6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   89.68%   89.74%   +0.06%     
==========================================
  Files         144      144              
  Lines       12291    12351      +60     
==========================================
+ Hits        11023    11085      +62     
+ Misses       1268     1266       -2     

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