webpack hooks API for SourceFileExtensionsPlugin
webpack hooks API for SourceFileExtensionsPlugin

This moves us toward resolution of the following warning:
  DeprecationWarning: Tapable.plugin is deprecated.
  Use new API on `.hooks` instead

Test plan:
 - tests pass

Closes FOO-2563

flag=none

Change-Id: I69429eac6e971268b722ed347d7b4a37473cd622
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/278023
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Ahmad Amireh <a53a33601b8dd9d06ae9e50f1f30fbe957aba866@instructure.com>
QA-Review: Ahmad Amireh <a53a33601b8dd9d06ae9e50f1f30fbe957aba866@instructure.com>
Product-Review: Ahmad Amireh <a53a33601b8dd9d06ae9e50f1f30fbe957aba866@instructure.com>

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