Implement JS source maps

[rnwst]

Source maps would greatly aid in debugging JS code included in the markdown document. Some useful resources on JS source maps:

• Article Source maps under the hood
• Utility to test source maps: https://sokra.github.io/source-map-visualization/
• The Source map spec

Mapping the JS output to the markdown source would rely on finding code strings in the source document, which cannot be done unambiguously if the same string is found more than once. To do this properly one would need AST source mapping, but this isn't currently implemented for pandoc's Markdown -- see jgm/pandoc#4565.