Add "flash" bind function to flash

This is something we do regularly in the reader, e.g. when there is no
completion.

This makes it accessible to custom bindings.

E.g. you can e.g. make the
alt-s binding (which toggles a sudo/doas/please prefix) flash instead
of doing anything when the current command is a builtin.

doc_src/cmds/bind.rst

@@ -218,6 +218,9 @@ The following special input functions are available:
 ``exit``
     exit the shell
 
+``flash``
+    makes the commandline flash, like fish does when e.g. there is no completion.
+
 ``forward-bigword``
     move one whitespace-delimited word to the right
 

src/input.rs

@@ -166,6 +166,7 @@ const INPUT_FUNCTION_METADATA: &[InputFunctionMetadata] = &[
     make_md(L!("execute"), ReadlineCmd::Execute),
     make_md(L!("exit"), ReadlineCmd::Exit),
     make_md(L!("expand-abbr"), ReadlineCmd::ExpandAbbr),
+    make_md(L!("flash"), ReadlineCmd::Flash),
     make_md(L!("force-repaint"), ReadlineCmd::ForceRepaint),
     make_md(L!("forward-bigword"), ReadlineCmd::ForwardBigword),
     make_md(L!("forward-char"), ReadlineCmd::ForwardChar),

src/input_common.rs

@@ -98,6 +98,7 @@ pub enum ReadlineCmd {
     TogglecaseChar,
     TogglecaseSelection,
     Execute,
+    Flash,
     BeginningOfBuffer,
     EndOfBuffer,
     RepaintMode,

src/reader.rs

@@ -2911,6 +2911,10 @@ impl<'a> Reader<'a> {
                         .reset_abandoning_line(usize::try_from(termsize_last().width).unwrap());
                 }
             }
+            rl::Flash => {
+                self.flash();
+                return;
+            }
             rl::HistoryPrefixSearchBackward
             | rl::HistoryPrefixSearchForward
             | rl::HistorySearchBackward