Add externs.js file to deal with name clashes between generated and external javascript

[ohri-anurag]

I have:

• Based work on latest develop branch
• Followed the contribution guide
• Looked for lint in my changes with hlint . (lint found code you did not write can be left alone) - Did not change Haskell code.
• Run the test suite: $(nix-build -A selftest --no-out-link)
• Updated the changelog
• (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[ohri-anurag]

Need to update the docs, will do it next week on Monday.

[alexfmpe]

I wonder if we should have a sample externs.js with commented contents used in the skeleton so it's more discoverable. Then the FAQ could just link to it.

[madeline-os]

This looks good. I agree that we should add an example to the skeleton. Once we have that, I will approve :)

[madeline-os]

After some discussion, we have decided that the best course of action is to fix this with automation before we bless a solution in the skeleton.