Feature Request: Variable preview should show extended variable, not buffer line

[yingzhu146]

Hi,

Thanks for this awesome plugin!

For variables, the preview is often to small. I'm less concerned about the position in the file (I often know that one), however container datastructures such as python dictionaries are most of the time way too large for the left hand pane. Thus I'd love to have the previewer show an unfolded representation of the variable.

If you don't have capacity, I could try to open a PR but would need a pointer as to how I could to return a serialized representation of the variables (I'm not entirely understanding how preview_fn could be used to just return a representation

[ToolmanP]

I guess we can leverage the tree_spec defined in dap.entity and eagerly fetch all the contents wrapped inside the variable and hook the response with the preview function to support this. I might file a PR for this later.