.html -> .svelte

[Rich-Harris]

#2067

[codecov-io]

Codecov Report

Merging #2071 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2071   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files           1       1           
  Lines          48      49    +1     
======================================
+ Hits           48      49    +1

Impacted Files	Coverage Δ
register.js	100% <100%> (ø)	⬆️

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[Rich-Harris]

Switched a couple of apps over (including v3.svelte.technology), updated the templates, and everything seems to continue to work, with either .html or .svelte extensions (though the REPL will automatically interpret .html files in gists as .svelte — seemed like the only practical way, even though it will mean cases where import declarations reference .html files that apparently don't exist), so... I guess it's time to pull the trigger?