feat(config): add "auto" position option for panels (sindrets#536)

Add support for position = "auto" in file_panel.win_config which
respects vim's splitright/splitbelow options when determining panel
placement. Users can set this to have the panel position follow their
vim split preferences.

Author: dlyongemallo

lua/diffview/ui/panel.lua

@@ -269,7 +269,17 @@ function Panel:open()
   local config = self:get_config()
 
   if config.type == "split" then
-    local split_dir = vim.tbl_contains({ "top", "left" }, config.position) and "aboveleft" or "belowright"
+    -- Resolve "auto" position based on vim's splitright/splitbelow options.
+    local position = config.position
+    if position == "auto" then
+      if self.state.form == "row" then
+        position = vim.o.splitbelow and "bottom" or "top"
+      else
+        position = vim.o.splitright and "right" or "left"
+      end
+    end
+
+    local split_dir = vim.tbl_contains({ "top", "left" }, position) and "aboveleft" or "belowright"
     local split_cmd = self.state.form == "row" and "sp" or "vsp"
     local rel_winid = config.relative == "win"
       and api.nvim_win_is_valid(config.win or -1)
@@ -282,7 +292,7 @@ function Panel:open()
       api.nvim_win_set_buf(self.winid, self.bufid)
 
       if config.relative == "editor" then
-        local dir = ({ left = "H", bottom = "J", top = "K", right = "L" })[config.position]
+        local dir = ({ left = "H", bottom = "J", top = "K", right = "L" })[position]
         vim.cmd("wincmd " .. dir)
         vim.cmd("wincmd =")
       end