assign custom methods to custom element prototype

[Rich-Harris]

fixes #1369

[codecov-io]

Codecov Report

Merging #1375 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1375   +/-   ##
=======================================
  Coverage   91.11%   91.11%           
=======================================
  Files         120      120           
  Lines        4421     4421           
  Branches     1360     1361    +1     
=======================================
  Hits         4028     4028           
  Misses        156      156           
  Partials      237      237

Impacted Files	Coverage Δ
src/compile/dom/index.ts	95.93% <100%> (ø)	⬆️

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