Add --esm switch that generates a project using ES modules by default.

Eliminate whitespace differences in the generated app.js.

Adding testing. Necessitated adding .js extension to generated bin/www filename.

Remove commented-out code.

Modify the --esm switch processing to error out of the generator if the Node version is less than 14. Update documentation and testing to reflect this change.

Fix esm 'should export an express app from app.js; test failures for early Node versions by moving import into a call to eval(). Also correct two errors in this test and include error handling.

In addition, update all generated code when --esm is active to use const rather than var and arrow syntax rather than function expression syntax for callbacks.

Change switch from esm to es6 to better reflect what the switch does.

Use parallel template files for non-es6 and es6 versions of app and www. In es6 mode, use .mjs extension on all generated JavaScript files. Remove node: in the es6 "should export" test's require of url.

The type field is now added to package.json only when the generator is run with the --es6 switch.

The node: URL scheme was not available in v14 until v14.13.1. So, removing this scheme to stay compatible with all releases of v14.

Per recommendation by aagamezl.