Document webpack change: fix: add typings for DefaultStatsFactoryPlugin, Stats, and MultiStats

[webpack-bot]

A pull request by @davidhouweling was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#12228

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What kind of change does this PR introduce?

Add typings matching the JSON output.

Note: the types for output are defined as Stats* because the types are merged together into the root level types.d.ts which might cause issues with other definitions. I know we could use namespaces, but I don't believe that is possible without fully converting to TypeScript itself.

Did you add tests for your changes?

No as it's primarily compile time based, so difficult to add tests. How I've tested this:

• Locally modified tests/Stats.test.js so that I captured the output of the toJson call into a file
• Locally apply the output to a JS file and do the type cast to verify it's correct.

Does this PR introduce a breaking change?

No.

What needs to be documented once your changes are merged?

In @types/webpack@^4 we would originally get the structure of the JSON output as webpack.Stats.ToJsonOutput.
In version 5, we would now do webpack.StatsCompilation.

[chenxsan]

We don't have any specific pages for those typings. And I believe IDEs should be able to give users the hint, hence no need to document it.