Automatically set module: true for the esm format

[NotWoods]

Terser has a special option module for minifying ES modules:

• module (default false) — Use when minifying an ES6 module. "use strict"
  is implied and names can be mangled on the top scope. If compress or
  mangle is enabled then the toplevel option will be enabled.

This change updates the plugin so that if the "esm" output format is detected, the module flag is turned on by default.

Since multiple output formats can be set, the serialized minifierOptions has been split into two separate variables - one for format: 'es' and one for other formats. The serialized versions are created lazily so that if only 1 format is used, only 1 variable will be set.

Thanks to this change, the "works with code splitting" test now creates smaller code since the strings can be inlined.

[TrySound]

Could you rebase from master and update tests?

[TrySound]

Thanks! Nice feature