Server-side code should be compiled for Node.js target

[ziir]

Somewhat related to:

• mozilla/addons-frontend#11593
• mozilla/addons#2162

At the moment, server-side code is compiled w/ Babel on the fly via @babel/register, while client-side code is compiled via babel-loader. For tests, all code is compiled via babel-jest transformer.

In all cases, the central Babel configuration file is used, which includes @babel/preset-env preset, leveraging the central .browserslistrc configuration file, which only lists browser targets today, regardless of compilation context or environment variables.

This leads to the server-side code being compiled for browser targets, whereas it runs in a Node.js context, which poses multiple issues on server-side code specifically:

• performs unnecessary browser-specific transformations, which slows down boot time
• over-polyfills the code, which may negatively impact runtime performance
• may not perform some Node.js specific necessary transforms, which may lead to bugs that will be exceptionally difficult to debug

This can be solved by either:

• leveraging BROWSERSLIST_ENV environment variable in .browserslistrc configuration file for the Node.js process
• leveraging BROWSERSLIST_CONFIG environment variable` to specify path of another Browserslist configuration file for the Node.js process
• preferably, compile server-side code ahead of time, and do not rely on @babel/register at runtime in production-like environments (#11626).

┆Issue is synchronized with this Jira Task

[KevinMind]

Old Jira Ticket: https://mozilla-hub.atlassian.net/browse/ADDFRNT-85