fix(plugin-webpack): add missing debug level for webpack-dev-middleware

[thateverdmitriy]

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Summarize your changes:

Tried to bootstrap new Electron app with elector-forge (TS + plugin-webpack). Got such error:

An unhandled exception has occurred inside Forge:
log.debug is not a function
TypeError: log.debug is not a function
    at /Users/project/node_modules/webpack-dev-middleware/lib/fs.js:63:21
    at /Users/project/node_modules/graceful-fs/graceful-fs.js:57:14
    at FSReqCallback.oncomplete (fs.js:153:23)

Thought it could be on my end and tried to create brand new project following setup guide:

$ yarn create electron-app my-new-app --template=webpack

It fail with same error as well. After checking where problem actually appears, found that webpack-dev-middleware has log.debug method in use: https://github.com/webpack/webpack-dev-middleware/blob/master/lib/fs.js#L63

In their README they mention debug as required logger level as well: https://github.com/webpack/webpack-dev-middleware#logger

plugin-webpack has debug logger level missing. One-line fix solving the issue.

[malept]

I wonder what changed in webpack-dev-middleware to cause this. I checked the changelog very briefly and didn't see anything.