add parserOptions.importMetaContext option

[vankop]

What kind of change does this PR introduce?
bugfix, add missing javascript parser option

Did you add tests for your changes?
updated existing

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
add parserOptions.importMetaContext. this option enables/disables import.meta.webpackContext evaluation.

[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)

[sokra]

Thanks

[webpack-bot]

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