Add click-header event for reporting clicks within header (#3768)

Sets $FZF_CLICK_HEADER_LINE and $FZF_CLICK_HEADER_COLUMN env vars with
coordinates of the last click inside and relative to the header and
fires click-header event.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

src/options.go

@@ -708,6 +708,8 @@ func parseKeyChordsImpl(str string, message string, exit func(string)) map[tui.E
 			add(tui.Jump)
 		case "jump-cancel":
 			add(tui.JumpCancel)
+		case "click-header":
+			add(tui.ClickHeader)
 		case "alt-enter", "alt-return":
 			chords[tui.CtrlAltKey('m')] = key
 		case "alt-space":