Update webpack to improve performance

[RDIL]

Speeds up the process.

Webpack changelogs

• disallow cache group named test with shorthand syntax to point out a potential config error
• Improve performance of LimitChunkCountPlugin
• fix a bug that the HMR plugin affected child compilations
• improve performance of splitChunks name option by caching hashed value
• improve rebuild performance by caching module size computation

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[mrmckeb]

I'm OK with this, but we're getting a lot of CI failures here - it completely fails on Windows.

Re-running checks ;)

[mrmckeb]

@ianschmitz any thoughts on why these are failing? I assume a simple Webpack version bump wouldn't be the issue...

[ianschmitz]

@ianschmitz any thoughts on why these are failing? I assume a simple Webpack version bump wouldn't be the issue...

Appears to be failing on master as well. Doesn't appear to be related to this PR. Bah silly Windows!

[mrmckeb]

Thanks for your help @RDIL!

And @ianschmitz, let's talk about this during the week. I think a lot of the tests could do with some love anyway.