Use GitHub URLs instead of local paths in SJSIR files

[fthomas]

This replaces local file paths in Scala.js SJSIR files with GitHub URLs.

It enables to see circe sources in the browser in Scala.js projects and gets rid of warnings like these in projects that use scalajs-bundler and webpack:

[warn] Module Warning (from ./node_modules/source-map-loader/index.js):
[warn] (Emitted value instead of an instance of Error) Cannot find source file
  '../../../../../../../../../../../../home/travis/code/projects/circe/modules/core/js/target/scala-2.13/src_managed/main/io/circe/TupleDecoders.scala'

Note that the same is done in Cats.

[codecov-io]

Codecov Report

Merging #1703 (839ca4e) into master (f144acd) will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1703      +/-   ##
==========================================
- Coverage   87.08%   86.99%   -0.09%     
==========================================
  Files          59       59              
  Lines        2392     2392              
  Branches      136      136              
==========================================
- Hits         2083     2081       -2     
- Misses        309      311       +2     

Impacted Files	Coverage Δ
...re/shared/src/main/scala/io/circe/MapDecoder.scala	88.00% <0.00%> (-8.00%)	⬇️
...rc/main/scala/io/circe/numbers/BiggerDecimal.scala	88.30% <0.00%> (+1.16%)	⬆️

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[travisbrown]

Thanks for catching this!