handle hashbangs in javascript files

[sokra]

implements https://github.com/tc39/proposal-hashbang
fixes vercel/next.js#27806

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?
nothing

[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]

@sokra The most important CI builds failed. This way your PR can't be merged.

Please take a look at the CI results from azure (1 errors / 0 warnings) and appveyor (success) and fix these issues.

[ctjlewis]

Great work, TYSM!