Allow bare put action in transform output

transform/bg-transform now permit bare `put`, inserting the key that
triggered the action (`a:transform:echo put` puts `a`).

src/tui/tui.go

@@ -4,6 +4,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unicode"
 
 	"github.com/junegunn/fzf/src/util"
 	"github.com/rivo/uniseg"
@@ -252,6 +253,12 @@ func (e Event) Comparable() Event {
 	return Event{e.Type, e.Char, nil}
 }
 
+// Printable returns true if the event is a printable character that can be
+// inserted into the query (e.g. via the 'put' action).
+func (e Event) Printable() bool {
+	return e.Type == Rune && unicode.IsGraphic(e.Char)
+}
+
 func (e Event) KeyName() string {
 	if me := e.MouseEvent; me != nil {
 		return me.Name()