feat: add completion to pre-filtering (#626)

Works by pressing `<C-l>` in insert mode. Supported are all builtins that have prefiltering. Means:
- lsp_workspace_symbols
- lsp_workspace_diagnostics
- lsp_document_symbols
- lsp_document_diagnostics

lua/telescope/mappings.lua

@@ -28,6 +28,7 @@ mappings.default_mappings = config.values.default_mappings or {
       ["<S-Tab>"] = actions.toggle_selection + actions.move_selection_better,
       ["<C-q>"] = actions.send_to_qflist + actions.open_qflist,
       ["<M-q>"] = actions.send_selected_to_qflist + actions.open_qflist,
+      ["<C-l>"] = actions.complete_tag
     },
 
     n = {