[DatePicker] Add missing exports

[havgry]

As agreed up in #23608, I've adding missing exports and cleaned up the index files a bit.

I've broken my changes up into quite a few commits to make it as easy as possible to understand what's going on in this PR. I ran into one issue which I'm not sure should be solved in this PR:

../material-ui-lab/src/index.d.ts(35,1): error TS2308: Module './DateRangePicker' has already exported a member named 'DateRange'. Consider explicitly re-exporting to resolve the ambiguity.
../material-ui-lab/src/index.d.ts(53,1): error TS2308: Module './DateRangePicker' has already exported a member named 'DateRange'. Consider explicitly re-exporting to resolve the ambiguity.
../material-ui-lab/src/index.d.ts(95,1): error TS2308: Module './DateRangePicker' has already exported a member named 'DateRange'. Consider explicitly re-exporting to resolve the ambiguity.
../material-ui-lab/src/index.d.ts(116,1): error TS2308: Module './PickersCalendarSkeleton' has already exported a member named 'styles'. Consider explicitly re-exporting to resolve the ambiguity.

• I have followed (at least) the PR section of the contributing guide.

Closes #23608

[mui-pr-bot]

@material-ui/lab: parsed: +0.23% , gzip: +0.75%

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