[plotly.js] fix types for image export

[fujidana]

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• Provide a URL to documentation or source code which provides context for the suggested changes:
  • [BUG]: imperfect type definitions around graph export features plotly/plotly.js#8057
    • https://plotly.com/javascript/plotlyjs-function-reference/#plotlytoimage
    • https://plotly.com/javascript/configuration-options/#customize-download-plot-options
• If this PR brings the type definitions up to date with a new version of the JS library, update the version number in the package.json.

---

This PR fixes the following type definition problems around Plotly.js's image export features:

1. Lack of "full-json" literal type in ToImageFormat union type.
2. Lack of setBackground property in ToImgopts and DownloadImgopts interfaces
3. Lack of imageDataOnly property in ToImgopts interface.
4. Several properties in ToImgopts and DownloadImgopts wrongly set as required.
5. The second parameter of downloadImage() wrongly set as required.

Actually full-json format option has been available since plotly.js v1.53.0 released 5 years ago (see also plotly/plotly.js#4593) but until recently it was not documented (see plotly/graphing-library-docs#471). Documentation about imageDataOnly was also updated then but it seems the feature has been also available from old versions. Now the updated documentation is available in the official Function Reference and Configuration Options documentation pages.

I checked full-json and imageDataOnly change the exported data as expected on both Plotly.js v2.25.2 and v3.7.0. Also, I confirmed that the properties in ToImgopts and DownloadImgopts are actually optional (without providing them, no error is thrown) on both versions. I can't find documentation about setBackground and don't know when it was added. Anyway, type definitions of this property will not be harmful for existing code since it is optional.

Plotly.js v4 started to provide the type definitions by themselves. Reflecting the doc updates mentioned above, theier type definition was also updated (plotly/plotly.js#8066). What this PR updates are semantically very close to what their PR does. Plotly.js v4 currently only bundles the types with the main package. Therefore, for users of a partial package such as plotly-basic-dist.min the type definition here is still important. So is for version 3 users, of course.

In the process of fixing the problems listed above, the following code changes have been made:

• Create SetBackground and ToImageFormat types and ToImageButtonOptions interface, instead of directly defining them in fields of other types/interfaces.
• Inherit ToImgopts and DownloadImgopts from ToImageButtonOptions, instead of redundantly defining their properties.

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1 package in this PR

• plotly.js (3.0) — on npm, on unpkg

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---

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