Improve usage documentation

[fregante]

After #376 is merged, I'll send another PR to simplify/condense the various import methods because currently each section is too long to follow.

#351 wasn't documented either and that's probably the easiest usage for most webpack users since it works across contexts.

The usage would be prefaced with a section like:

• without a bundler:
  • static: add browser-polyfill.js to manifest.json
  • dynamic: inject file via executeScript before other files
• bundler usage:
  • simple: use const browser = require('webextension-polyfill')
  • invisible webpack usage: use ProvidePlugin
  • polyfill the global: use require('webextension-polyfill/register')

And the ESM usage section should be dropped because there's nothing ESM-related about it, type=module effectively doesn't change anything here. It's also rather confusing because the example is an HTML page but it does not mention how to load such HTML page (which is also deprecated)