feat: custom window selection

See #212

doc/nvim-dap-ui.txt

@@ -266,6 +266,8 @@ Layouts are opened in the order defined
 {controls} `(dapui.Config.controls)` Controls configuration
 {render} `(dapui.Config.render)` Rendering options which can be updated
 after initial setup
+{select_window?} `(fun(): integer)` A function which returns a window to be
+used for opening buffers such as a stack frame location.
 
                                                             *dapui.Config.icons*
 Fields~

lua/dapui/config/init.lua

@@ -17,6 +17,8 @@ local dapui = {}
 ---@field controls dapui.Config.controls Controls configuration
 ---@field render dapui.Config.render Rendering options which can be updated
 --- after initial setup
+---@field select_window? fun(): integer A function which returns a window to be
+--- used for opening buffers such as a stack frame location.
 
 ---@class dapui.Config.icons
 ---@field expanded string

lua/dapui/util.lua

@@ -96,6 +96,9 @@ local function set_opts(win, opts)
 end
 
 function M.select_win()
+  if config.select_window then
+    return config.select_window()
+  end
   local windows = vim.tbl_filter(function(win)
     if api.nvim_win_get_config(win).relative ~= "" then
       return false