support custom events on <:Window>

[Rich-Harris]

fixes #1268.

Again, I'm being slightly lazy about DRY because I feel a big refactoring coming on. Will throw up a new issue describing what I mean

[codecov-io]

Codecov Report

Merging #1315 into master will decrease coverage by 0.02%.
The diff coverage is 80%.

@@            Coverage Diff            @@
##           master   #1315      +/-   ##
=========================================
- Coverage   91.53%   91.5%   -0.03%     
=========================================
  Files         124     124              
  Lines        4419    4427       +8     
  Branches     1391    1393       +2     
=========================================
+ Hits         4045    4051       +6     
- Misses        154     155       +1     
- Partials      220     221       +1

Impacted Files	Coverage Δ
src/validate/html/validateWindow.ts	88.23% <100%> (+0.73%)	⬆️
src/generators/nodes/Window.ts	80% <77.77%> (-0.89%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8717ff8...6ef808c. Read the comment docs.