Add new treeshaking options

[lukastaegert]

This PR will move options.pureExternalModules to options.treeshake.pureExternalModules and make it configurable via config file and also the CLI via --treeshake.pureExternalModules/--no-treeshake.pureExternalModules.

It also adds the new options.treeshake.propertyReadSideEffects which defaults to true but can be disabled by setting it to false. If it is disabled, the following side-effects will not be detected and result will be removed if it is not used elsewhere in the code:

• accessing a getter with side-effects

  const foo = {
    get bar() {
      console.log('effect');
      return 'bar';
    }
  }
  
  const result = foo.bar;

• accessing members of null or undefined:

  const foo = {}
  const result = foo.bar.baz;

It is my guess that setting options.treeshake.propertyReadSideEffects: false or using --no-treeshake.propertyReadSideEffects will work without problems in 95% of the use cases and can noticably reduce bundle size especially when passing around a lot of objects as function parameters.

Also, a lot of unnecessary property access checks were removed in this PR which should give a slight speed boost.

Of course we should not forget to update the documentation once this is released.

[kzc]

Course-grained propertyReadSideEffects should be fine for most scenarios.

Are unknown globals still considered side effect free in all cases?

[lukastaegert]

Yes, for now you cannot use try..catch to check for the existence of globals 😉 As this is the previous behaviour and there are no open issues regarding this yet, I decided to postpone this for a later release.