add webpack config file, tell webpack about mjs files

[micahstubbs]

this PR will fix the webpack error about .mjs JavaScript modules files. 😄

short story: it looks like we cannot consume javascript modules (.mjs) files in webpack 4 without telling webpack how to parse those files. the solution that I suggest adds a webpack.config.js file that tells webpack how to parse the .mjs file that d3-circular-sankey ships.

an example to demonstrate a client-code fix for tomshanley/d3-sankey-circular#32

[micahstubbs]

@peteruithoven I'll share my research process, to help you next time 😄

1. I google searched the error message Can't import the named export 'ascending' from non EcmaScript module https://www.google.com/search?q=Can%27t+import+the+named+export+%27ascending%27+from+non+EcmaScript+module+(only+default+export+is+available)&oq=Can%27t+import+the+named+export+%27ascending%27+from+non+EcmaScript+module+(only+default+export+is+available)&aqs=chrome..69i57.333j0j7&sourceid=chrome&ie=UTF-8

[micahstubbs]

2. I read the top 2 search results, and picked the top rated comments.

apollographql/apollo-link-state#302 (comment)

and

graphql/graphql-js#1272 (comment)

[micahstubbs]

3. I realized that I need to generalize from a Vue framework config file wrapped a webpack config in that first comment that to just a plain webpack config, so I went and read the webpack 4 docs https://webpack.js.org/configuration/

[micahstubbs]

4. then I tried the resulting webpack config with this demo project, and et voila, it worked 🎉

build text

➜ d3-sankey-circular-test git:(micah/exp/fix-import) ✗ npm run build

[email protected] build /Users/m/workspace/d3-sankey-circular-test
webpack

Hash: 4aa38395611a79ec6eed
Version: webpack 4.29.5
Time: 295ms
Built at: 02/24/2019 6:46:56 PM
Asset Size Chunks Chunk Names
main.js 59.2 KiB 0 [emitted] main
Entrypoint main = main.js
[5] ./src/index.js + 1 modules 55.4 KiB {0} [built]
| ./src/index.js 67 bytes [built]
| + 1 hidden module
[6] ./node_modules/d3-path/src/index.js + 1 modules 4.12 KiB {0} [built]
| 2 modules
+ 5 hidden modules

[tomshanley]

Micah - you're a star. thanks for making this update. I tried reading this article but got lost some where in it https://medium.com/passpill-project/files-with-mjs-extension-for-javascript-modules-ced195d7c84a