feat: support "..." in resolve importsFields

[ahabhgk]

What kind of change does this PR introduce?

feat or fix, before using importsFields: ["custom", "..."] won't work, since there isn't a default value for importsFields

Did you add tests for your changes?

added

Does this PR introduce a breaking change?

No

What needs to be documented once your changes are merged?

mention supports "..." in resolve importsFields (if needed)

[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)