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.
+