TypeScript compiler needs to be configured to output ES6 module by
setting --module es6 or the equivalent tsconfig.json compilerOptions
parameter.
Since tsc --build does not accept compiler options flags, we need to
duplicate some of the configuration:
- specify separate output folder for ES6 module files (new
tsconfig.esm.json file)
- add "module" field for ES6 module (esm) output files to package.json
Hence, a script `scripts/sync-esm-config.js` was added to automate this
process.
