Make webpack optional
Make webpack optional

webpack checks imports for correctness -- but only if that import is
used. See previous commit.

`./build` for building with just Fable, `./build -t Webpack` to use
webpack.
Nice side effect: `yarn install` and downloading all `node_modules` is
only needed for webpack -- not for Fable only builds.
And: Fable without webpack is faster.

ionide/ionide-vscode-helpers,ionide/ionide-vscode-helpers