New "bookmark" plugin allows hotkey fast-travel to specific steps (#852)

add plugin bookmark supporting direct access via hotkeys
similar to "click", we can now fast travel using hotkeys e.g. 1 2 3

src/plugins/bookmark/README.md

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+# Bookmark
+
+Nonlinear navigation similar to the Goto plugin.
+
+Goto supports nonlinear navigation by *locally* defining *out-links*, accessible via the arrow keys.
+
+Bookmark supports nonlinear navigation by *globally* defining *in-links*, accessible via normal keys like 1,2,3,A,B,C.
+
+Example:
+
+```html
+<!-- data-bookmark-key-list allows an "inbound"-oriented style of non-linear navigation. -->
+<div id="..." class="step" data-bookmark-key-list="Digit1 KeyA 1 2 3 a b c">
+```
+
+An `id` is required on the `div`.
+
+If you assign the same key to multiple steps, that hotkey will cycle among them.
+
+WARNING: It's up to you to avoid reserved hotkeys H, B, P, ?, etc.
+
+Author
+------
+
+Copyright 2023 Wong Meng Weng (@mengwong)
+Released under the MIT license.
+

src/plugins/bookmark/bookmark.js

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+/**
+ * Bookmark Plugin
+ *
+ * The bookmark plugin consists of
+ *   a pre-init plugin,
+ *   a keyup listener, and
+ *   a pre-stepleave plugin.
+ *
+ * The pre-init plugin surveys all step divs to set up bookmark keybindings.
+ * The pre-stepleave plugin alters the destination when a bookmark hotkey is pressed.
+ *
+ * Example:
+ *
+ *       <!-- data-bookmark-key-list allows an "inbound" style of non-linear navigation. -->
+ *       <div id="..." class="step" data-bookmark-key-list="Digit1 KeyA 1 2 3 a b c">
+ *
+ * See https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/key/Key_Values for a table
+ * of what strings to use for each key. Both .key and .code styles are recognized.
+ *
+ * It's up to the HTML author to avoid reserved hotkeys H, B, P, ? etc.
+ *
+ * Copyright 2016-2017 Henrik Ingo (@henrikingo)
+ * Released under the MIT license.
+ */
+/* global document, impress */
+
+( function( document ) {
+    "use strict";
+    var hotkeys = {};
+    function hotkeyDest( event ) {
+	return ( hotkeys.hasOwnProperty( event.key )  ? hotkeys[ event.key ] :
+		 hotkeys.hasOwnProperty( event.code ) ? hotkeys[ event.code ] : null ); }
+
+    // In pre-init phase, build a map of bookmark hotkey to div id, by reviewing all steps
+    impress.addPreInitPlugin( function( root, api ) {
+	root.querySelectorAll( ".step" ).forEach( function( div ) {
+            if ( div.dataset.bookmarkKeyList !== undefined && div.id !== undefined ) {
+		div.dataset.bookmarkKeyList.split( " " ).forEach( ( k ) => {
+		    if ( hotkeys.hasOwnProperty( k ) ) {
+			hotkeys[ k ].push( div.id );
+		    } else { hotkeys[ k ] = [ div.id ]; } } ); } } );
+
+	api.lib.gc.addEventListener( document, "keyup", function( event ) {
+	    if ( hotkeyDest( event ) !== null ) {
+		event.stopImmediatePropagation();
+		api.next( event );
+
+		// Event.preventDefault();
+	    }
+	} );
+    } );
+
+    // In pre-stepleave phase, match a hotkey and reset destination accordingly.
+    impress.addPreStepLeavePlugin( function( event ) {
+
+	// Window.console.log(`bookmark: running as PreStepLeavePlugin; event=`);
+	// window.console.log(event)
+        if ( ( !event || !event.origEvent ) ) { return; }
+	var dest = hotkeyDest( event.origEvent );
+        if ( dest ) {
+
+	    // Window.console.log(`bookmark: recognizing hotkey ${event.code} goes to ${dest}`)
+            var newTarget = document.getElementById( dest[ 0 ] ); // jshint ignore:line
+            if ( newTarget ) {
+                event.detail.next = newTarget;
+		dest.push( dest.shift() ); // Repeated hotkey presses cycle through each dest.
+            }
+        }
+    } );
+
+} )( document );
+