feat: Add support for select.clipboard (#304)

Useful for providing system clipboard contents easily (for example for
using CopilotChat quickly in replace mode in tmux popup etc)

Signed-off-by: Tomas Slusny <[email protected]>

lua/CopilotChat/select.lua

@@ -49,7 +49,7 @@ function M.visual(source)
   return get_selection_lines(bufnr, start_line, start_col, finish_line, finish_col, false)
 end
 
---- Select and process contents of unnamed register ('"')
+--- Select and process contents of unnamed register ("). This register contains last deleted, changed or yanked content.
 --- @return CopilotChat.config.selection|nil
 function M.unnamed()
   local lines = vim.fn.getreg('"')
@@ -63,6 +63,20 @@ function M.unnamed()
   }
 end
 
+--- Select and process contents of plus register (+). This register is synchronized with system clipboard.
+--- @return CopilotChat.config.selection|nil
+function M.clipboard()
+  local lines = vim.fn.getreg('+')
+
+  if not lines or lines == '' then
+    return nil
+  end
+
+  return {
+    lines = lines,
+  }
+end
+
 --- Select and process whole buffer
 --- @param source CopilotChat.config.source
 --- @return CopilotChat.config.selection|nil