GitLab

minor QOL mainly for me; put webpack-specific configuration in its own
folder, make it portable, and make room for esbuild

the __webpack_public_path__ initializer no longer sources the value from
the build module, instead it uses a pre-defined global that webpack
injects

~ test plan ~

build is OK

Change-Id: I4ba4a3c0cb9175f96096f2b78022e152c04fc75d
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/276183

Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Charley Kline <ckline@instructure.com>
QA-Review: Charley Kline <ckline@instructure.com>
Product-Review: Charley Kline <ckline@instructure.com>