Switched to named imports

[radekmie]

While working on #624 I've found two problems:

• Themes importing uniforms package were importing certain files. This resulted in the website project importing ES6 build, but files inside were importing ES5 build of the core package.
• Docusaurus is using ES6 bundle, but it transpiles classes anyway (and in a different way than our ES5 build).

To fix both I've changed all the inter-package imports (both in sources and tests) to named ones. It allows the bundler to choose which build to include. In general, we should start to encourage such a way of importing now, as our ES6 build is already there.

Then I've encountered a problem with core-js in Docusaurus, so I've updated it as well. To preserve current sidebar behavior I've added sidebarCollapsible: false.

One more, not related issue: uniforms-material was not exporting wrapField as a named export.

[codecov]

Codecov Report

Merging #629 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #629   +/-   ##
======================================
  Coverage    99.6%   99.6%           
======================================
  Files         163     163           
  Lines        1782    1782           
  Branches      547     547           
======================================
  Hits         1775    1775           
  Misses          1       1           
  Partials        6       6

Impacted Files	Coverage Δ
packages/uniforms-material/src/DateField.tsx	100% <ø> (ø)	⬆️
packages/uniforms-bootstrap4/src/NestField.tsx	100% <ø> (ø)	⬆️
packages/uniforms-bootstrap4/src/wrapField.tsx	100% <ø> (ø)	⬆️
packages/uniforms-bootstrap4/src/LongTextField.tsx	100% <ø> (ø)	⬆️
packages/uniforms-antd/src/QuickForm.tsx	100% <ø> (ø)	⬆️
packages/uniforms-antd/src/ListAddField.tsx	100% <ø> (ø)	⬆️
packages/uniforms-material/src/ErrorsField.tsx	100% <ø> (ø)	⬆️
packages/uniforms-bootstrap4/src/ErrorField.tsx	100% <ø> (ø)	⬆️
packages/uniforms-semantic/src/QuickForm.tsx	100% <ø> (ø)	⬆️
packages/uniforms-semantic/src/ListDelField.tsx	100% <ø> (ø)	⬆️
... and 124 more

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