[ui][enhancement] Improve dropdown for dataset example picker

[rickarize]

The current dataset example picker shows preview data that is flattened into an unweildy string and shown identically across all states.

• Add more readable line numbers / indices (these should be for display only, because as dataset rows get added/removed, the index may change, so pinning to index is unsafe)
• Consider dropping the substring that's common across all examples. This might leave a single differing char, or an uneven number of remaining chars, per example, which is odd.

* Fix wrap behavior. The checkmark is at the far right of the list item, pushed off screen. My preference is for pre-allocated left aligned checkboxes, but we should be consistent with the codebase which is probably right-aligned wrapping for now. * Add truncate behavior. Dataset content can be exceptionally long and makes for a tiny horizontal scrollbar. * There's no preview when hovering over the unopened picker, but that's the correct behavior IMO. The JSON editor is its own preview, and we should support better preview when hovering over a specific item.

Here's a shot of the original mocks 1. 2 This spends most of the real estate on selecting an alternate dataset, which doesn't match how usage has evolved (away from dataset selection). Still, we could show this in a fixed state.

[mikeldking]

@rickarize the problem here is that you need some sort of preview of the element. Without this, it's impossible to select examples.