Use List instead of Tree for the QuickPick

[isidorn]

Since QuickPick is a flat list It does not make sense to use the Tree as the implenetation mechanism we should use the List instead.
Refs: #27900

[alexdima]

@isidorn Not sure how these widgets all work, but fyi in the standalone editor they are located in src/vs/editor/standalone/browser/quickOpen

[isidorn]

@bpasero I might look into this, any objections to assigning this to November?

[bpasero]

@isidorn sure feel free. this would probably have to go into quickOpenWidget.ts. I hope the list covers all the features we have from the tree...

[brunnerh]

Does the list implementation have virtualization support?

I just wrote an extension where ideally one could search through about 30,000 items. Currently this is not feasible in terms of performance; the item elements probably would have to be created only when they are actually visible.

[bpasero]

@chrmarti can answer to that as he changed the underlying widget for the extensions picker just recently and I think this should improve performance at least for extensions.

[chrmarti]

Performance has improved with the list widget, but there is no virtualization support in the API at the moment.

There are a few cases where we still use the old implementation internally, the API always uses the list widget. Keeping this task around for the remaining cases.

[bpasero]

/duplicate #69955

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