webpack: add child context properties

[LukeSheard]

Adds missing properties to a "Compiler" instance as seen in following lines of code:

https://github.com/webpack/webpack/blob/master/lib/Compiler.js#L183
https://github.com/webpack/webpack/blob/master/lib/Compiler.js#L565
https://github.com/webpack/webpack/blob/master/lib/Compiler.js#L122

[typescript-bot]

@LukeSheard Thank you for submitting this PR!

🔔 @tkqubo @bumbleblym @bcherny @tommytroylin @mohsen1 @jcreamer898 @alan-agius4 @elliottsj @jason0x43 @dennispg @christophehurpeau @ZSkycat @johnnyreilly @rwaskiewicz @kuehlein @grgur @rubenspgcavalcante - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[johnnyreilly]

Thanks @LukeSheard!

[typescript-bot]

I just published @types/webpack@4.4.28 to npm.