don't transform bidi transitions twice

[Rich-Harris]

fixes the regression in #962 that would fail on bidirectional transitions that referenced state

[codecov-io]

Codecov Report

Merging #970 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #970      +/-   ##
==========================================
+ Coverage   91.84%   91.86%   +0.02%     
==========================================
  Files         112      112              
  Lines        4107     4108       +1     
  Branches     1332     1330       -2     
==========================================
+ Hits         3772     3774       +2     
  Misses        143      143              
+ Partials      192      191       -1

Impacted Files	Coverage Δ
.../generators/dom/visitors/Element/addTransitions.ts	96.66% <100%> (+3.56%)	⬆️

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