Fix repl expressions leave an empty prompt in the middle (#1052)

[ktakiman]

Here is a few things I learned along the way and how I approached fixing #1052.

• REPL buffer is based on buftype=prompt, where nvim makes sure that there is always a prompt at the end of buffer in insert mode (dap> ).
• When a new command is entered, exec function in repl.lua is executed (registered via vim.fn.prompt_setcallback). nvim does not put the next prompt until callback returns, so, within the callback, new lines can simply be appendend at the end of the buffer. (e.g. .help command does not leave an empty prompt in the middle).
• Things get a bit tricky with expression command because when a DAP server request comes back, nvim-dap is aldready outside of the callback, and there is an empty prompt at the end of the buffer. Two options here so not to leave an empty prompt in the middle,
  1. insert the new line(s) right in front of the empty prompt
  2. overwrite the last line with the new line(s)
• append function inside repl.lua is updated to apply the first option. evaluate_handler now calls this function instead of layer.render where possible.
• The second opton is used where tree.render is called inside evaluate_handler.

[ktakiman]

Tested REPL window with lua (jbyuki/one-small-step-for-vimkind), c/c++ (codelldb), python(debugpy) and javascript (vscode-node-debug2) with this update, and so far not seeing any side effect and a blank prompt stopped appearing.

[mfussenegger]

Thanks for the great analysis and explanation.

[wookayin]

Amazing, thank you @ktakiman!