Produce esm modules during prepublish scripts

[evansiroky]

This fixes #65 by adding a separate build script that will produce esm modules that can be imported by other packages using the module entry point in the package.json file.

Specifically, the changes include:

• a new script build:esm is added
• linting and jest ignores these newly created esm directories in each package
• The babel config file is changed to facilitate the production of the esm files.
• Each package now has a module entry point
• All imports of packages using a path to some \lib\ directory are changed to import from the root export. Some packages were modified to better export variables to facilitate this conversion.

[binh-dam-ibigroup]

With the changes in this PR, do we also want to deprecate the use of destructured declarations of module exports such as in this example, whether they occur in otp-ui or otp-react-redux?

[binh-dam-ibigroup]

Looks good. Two thoughts:

• It doesn't look like any changes are breaking as the now-preferred alternative has always been available, but others can override me on that.
• We could use destructuring declarations such as these for methods that we use most often in a file, but I don't feel strongly about that.

[binh-dam-ibigroup]

This import will need to be updated in OTP-RR, so should this be noted as breaking?

[evansiroky]

I don't believe this is breaking since the lib folder is still created during the prepublish script. Therefore, these imports should still work. These imports of course wouldn't use the files from the esm folder, but things should just work when importing from package root.

[landonreed]

@evansiroky, could you provide some more info on how to leverage this approach and what it offers in the project README?

[evansiroky]

@evansiroky, could you provide some more info on how to leverage this approach and what it offers in the project README?

I don't think this is worthy of an entry within the README. In various other 3rd-party packages that produce various kinds of files, they don't bother mentioning it (see auth0-react, connected-react-router and yup). This module thing "just works" when using a bundler that is aware of modules which should be webpack 2+. If you copy the generated esm folder and package.json for one of these packages and add/replace the folder/file into the respective node_modules directory in trimet-mod-otp and then put in a log statement, you'll be able to confirm that webpack loads the esm file. The benefits of this are that webpack can do tree shaking because these files in the esm folders use the import/export syntax (see the first paragraph of the webpack doc page about tree shaking).