allow parameterised transitions

[Rich-Harris]

Fixes #962. I managed to break parameterised transitions in #924 😬

[codecov-io]

Codecov Report

Merging #963 into master will increase coverage by 0.03%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #963      +/-   ##
==========================================
+ Coverage   91.73%   91.76%   +0.03%     
==========================================
  Files         109      109              
  Lines        4051     4054       +3     
  Branches     1300     1301       +1     
==========================================
+ Hits         3716     3720       +4     
  Misses        150      150              
+ Partials      185      184       -1

Impacted Files	Coverage Δ
src/generators/Generator.ts	93.93% <100%> (+0.05%)	⬆️
.../generators/dom/visitors/Element/addTransitions.ts	93.1% <33.33%> (+3.44%)	⬆️

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