fix: use a single version of electron-forge

[bendemboski]

We have two versions of electron-forge kicking around. ember-electron has it as a dependency so it can use it from the blueprint to create the Electron project using electron-forge's init command. When electron-forge creates a project, it installs itself in that project, so we end up with another version in electron-app/package.json.

When ember-electron was calling electron forge commands from ember electron:* commands, it was just require()ing electron-forge, which would get it the version in /node_modules. This could be a different version from the one in electron-app/node_modules, so the user could get different behavior running, e.g. ember electron:make vs. cd electron-app && yarn electron-forge make, breaking the simple mental model.

Also, recently electron-forge released a new beta version that changed the maker interface slighty, so the makers installed in electron-app/ could be out of sync with the electron-forge version that the ember electron commands use. This is probably only a problem while electron-forge is in beta, but is also addressed by this change.

Now, other than the initial Electron project creation that happens from the blueprint, we only use the electron-forge in electron-app, which puts it right next to the makes in package.json allowing the user to easily see and manage the version without any hidden complexity.

Fixes #431

[jacobq]

Summarizing to check my understanding:

1. Motivation: ember-electron should avoid using its own copy (local node_modules) of any electron-forge packages (@electron-forge/*) to avoid discrepancies (e.g. version mismatch) / improve consistency (e.g. effectively the same behavior with ember electron:... as with equivalent forge commands in electron-app).
2. New file lib/utils/forge-core.js wraps calls to require('@electron-forge/core') and forces use of version in electron-app/node_modules/ (or GLOBAL_FOLDERS) -- throws an error if it's not there.
3. This behavior is overridden with a stub during tests
4. When the blueprints are run (e.g. during ember install ember-electron), ember-electron still uses the electron-forge API directly since (presumably) there isn't anything else yet.

   ember-electron/blueprints/ember-electron/index.js

   Line 2 in 8605e08

   const { api } = require('@electron-forge/core');