Add a tree shaking test that involves the use of Flow types

[AlexMSmithCA]

As part of the investigation into looking into WPT-2994 this PR adds a new test to ensure that imports/exports that include Flow types are properly stripped and tree shaken.

[AlexMSmithCA]

Lines 51-52 here cause the vendor bundle to leak the unused import from fixture-pckg (__FIXTURE_DEPENDENCY_UNUSED__ which should be used only in the server bundle):

/*! no static exports found */function(e,o,s){"use strict";Object.defineProperty(o,"__esModule",{value:!0});o.usedDependencyValue="__FIXTURE_DEPENDENCY_USED__",o.unusedDependencyValue="__FIXTURE_DEPENDENCY_UNUSED__"}}]);

Removing these two lines tree shakes the import for fixture-pckg.

[AlexMSmithCA]

We couldn't have export type declarations in the dependency's entry point, so maybe this test isn't the best...

Currently using the transpiled index.js here and added an index.js.flow file for Flow/IDE integration.

[AlexMSmithCA]

I was having trouble reproducing whether code was leaking into the browser due to Flow imports:

e.g.

import type {AType} from ... // no leak
import {B} from ...

vs.

import {type AType, B} from ... // potential leak

Not entirely sure why, but tree splitting seems to be failing for node module dependency imports as a separate concern.