Add undo

Add the input function undo which is bound to `\c_` (control + / on
some terminals). Redoing the most recent chain of undos is supported,
redo is bound to `\e/` for now.

Closes #1367.
This approach should not have the issues discussed in #5897.

Every single modification to the commandline can be undone individually,
except for adjacent single-character inserts, which are coalesced,
so they can be reverted with a single undo. Coalescing is not done for
space characters, so each word can be undone separately.

When moving between history search entries, only the current history
search entry is reachable via the undo history. This allows to go back
to the original search string with a single undo, or by pressing the
escape key.
Similarly, when moving between pager entries, only the most recent
selection in the pager can be undone.

src/input.cpp

@@ -145,7 +145,10 @@ static const input_function_metadata_t input_function_metadata[] = {
     {readline_cmd_t::func_and, L"and"},
     {readline_cmd_t::expand_abbr, L"expand-abbr"},
     {readline_cmd_t::delete_or_exit, L"delete-or-exit"},
-    {readline_cmd_t::cancel, L"cancel"}};
+    {readline_cmd_t::cancel, L"cancel"},
+    {readline_cmd_t::undo, L"undo"},
+    {readline_cmd_t::redo, L"redo"},
+};
 
 static_assert(sizeof(input_function_metadata) / sizeof(input_function_metadata[0]) ==
                   input_function_count,