webpack loaders: implement `emitError` and `emitWarning` (#3983)
This implements webpack loader context api for `emitError` and
`emitWarning`, each of which emit Turbopack issues at the appropriate
error level. As with webpack’s implementation, `emitError` does _not_
cause builds to fail [0].
This uses next-dev issue snapshots (#3774) to test that issues are
emitted.
[0] https://webpack.js.org/api/loaders/#thisemiterror
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Co-authored-by: Justin Ridgewell <justin@ridgewell.name>
