Add a new option dap.repl.show_tree: whether to show the tree in REPL

[wookayin]

A new option show_tree can be set by users to configure whether to
show the expandable tree in the DAP REPL window. Concise and simple
representation in the REPL would be useful for many languages,
see #737 for an example (python).

The option can be configured using, for instance:

require('dap.repl').show_tree = false

TODO: Add documentation in doc/dap.txt

[wookayin]

@mfussenegger Kindly bumping up and bringing this again to your attention.

[mfussenegger]

Thanks for the PR. I think having more options to control how the variables are displayed is something I want to add but this is rather limited and I'm not sure about the UI/UX.

I'll need to give this some more thought but I'm pretty sure it will be different than this, so I'm closing the PR

[wookayin]

Hope this or similar features can be implemented soon -- this is so essential for python debugging. I'm happy to discuss and contribute more.

[wookayin]

The reason I chose somewhat limited configuration like this is that nvim-dap doesn't have an universal configuration API, and previously we used to inject user configs into global variables (e.g. https://github.com/mfussenegger/nvim-dap/blob/master/doc/dap.txt#L564-L570, or injecting repl.commands.custtom_commands, etc.) We wouldn't need a UI for configuration.