add asset info for sourceFilename and javascriptModule

[sokra]

What kind of change does this PR introduce?
feature

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
two more properties in info (https://webpack.js.org/api/stats/#asset-objects)

• sourceFilename when asset was created from a source file (potentially transformed), the original filename relative to compilation context
• javascriptModule true, when asset is javascript and an ESM

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[webpack-bot]

I've created an issue to document this in webpack/webpack.js.org.