feat(context): mention @quickfix to add files in qf to context (#988)

Neovim allows quickfix list to be populated in a variety of ways: grep,
lsp symbol references etc. Being able to add files in the quickfix
window to the LLM chat context allows for interesting workflows. For
example, one could search for a symbol using the LSP integration,
populate the quickfix with that list and then pass those along as
context in Avante using @quickfix mention in the sidebar.

If there are no files in the quickfix list or the items do not have a
file, nothing is added to the context.

lua/avante/sidebar.lua

@@ -1803,6 +1803,13 @@ function Sidebar:create_input_container(opts)
           callback = function() self.file_selector:open() end,
         })
 
+        table.insert(mentions, {
+          description = "quickfix",
+          command = "quickfix",
+          details = "add files in quickfix list to chat context",
+          callback = function() self.file_selector:add_quickfix_files() end,
+        })
+
         cmp.register_source(
           "avante_commands",
           require("cmp_avante.commands"):new(self:get_commands(), self.input_container.bufnr)