Add CellOutputViewer for SandboxOutputs (#686)

* Initial commit

* Optimizations

* Optimize notebookOutputViewer bundle size by lazy loading transforms

* Update package-lock.json

* More optimizations

* Updates

* Fix unit test and other updates

* Address feedback

* Update package-lock.json

* Update test snapshots

* Fix build

* Reduce cellOutputViewer bundle size

* Renaming

src/CellOutputViewer/transforms/Vega2.ts

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+export { Vega2 as default } from "@nteract/transform-vega";

src/CellOutputViewer/transforms/Vega3.ts

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+export { Vega3 as default } from "@nteract/transform-vega";

src/CellOutputViewer/transforms/Vega4.ts

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+export { Vega4 as default } from "@nteract/transform-vega";

src/CellOutputViewer/transforms/Vega5.ts

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+export { Vega5 as default } from "@nteract/transform-vega";

src/CellOutputViewer/transforms/VegaLite1.ts

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+export { VegaLite1 as default } from "@nteract/transform-vega";

src/CellOutputViewer/transforms/VegaLite2.ts

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+export { VegaLite2 as default } from "@nteract/transform-vega";

src/CellOutputViewer/transforms/VegaLite3.ts

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+export { VegaLite3 as default } from "@nteract/transform-vega";

src/CellOutputViewer/transforms/VegaLite4.ts

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+export { VegaLite4 as default } from "@nteract/transform-vega";

src/CellOutputViewer/transforms/WidgetDisplay.ts

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+export { WidgetDisplay as default } from "@nteract/jupyter-widgets";