Add compileSrc option to InjectManifet webpack plugin

[jeffposnick]

R: @philipwalton
CC: @bailnl

Fixes #2383

When compileSrc is set to false, the swSrc file won't be run through the webpack compilation process. That allows for the use case described in #2383, where the manifest ends up injected into an arbitrary file that might not be something that should be processed by webpack.

The default behavior has not changed.

[bailnl]

thanks ~ @jeffposnick