Pin package.elm-lang.org version (tag) to 0.18.0

[ento]

package.elm-lang.org has been upgraded to 0.19, which doesn't have the expected project structure as 0.18.

[codecov-io]

Codecov Report

Merging #11 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #11   +/-   ##
=======================================
  Coverage   97.92%   97.92%           
=======================================
  Files          11       11           
  Lines         338      338           
=======================================
  Hits          331      331           
  Misses          7        7

Impacted Files	Coverage Δ
src/elm_doc/asset_tasks.py	97.56% <100%> (ø)	⬆️

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