Introduce "innerwiki" plugin

From the readme:

This plugin enables TiddlyWiki to embed a modified copy of itself (an "innerwiki"). The primary motivation is to be able to produce screenshot illustrations that are automatically up-to-date with the appearance of TiddlyWiki as it changes over time, or to produce the same screenshot in different languages

plugins/tiddlywiki/innerwiki/doc/examples.tid

@@ -0,0 +1,74 @@
+title: $:/plugins/tiddlywiki/innerwiki/examples
+
+\define example(text)
+<$codeblock code=<<__text__>>/>
+
+Renders as:
+
+$text$
+\end
+
+!! Browser
+
+The innerwiki widget specifies the dimensions of the virtual screen used to render the wiki (in pixels) and CSS styles to apply to it. Nested `<$data>` widgets are used to specify individual payload tiddlers to be loaded into the wiki. In this example, we initialise the innerwiki with two tiddlers "HelloThere" and "$:/DefaultTiddlers":
+
+<<example """<$innerwiki width="1200" height="400" style="width:100%;" filename="screenshot-1">
+	<$data title="HelloThere" text="This tiddler is inside a wiki"/>
+	<$data title="$:/DefaultTiddlers" text="HelloThere"/>
+</$innerwiki>""">>
+
+Note that the "screenshot" is a shrunken but fully interactive TiddlyWiki.
+
+!! Node.js
+
+To render these examples as a PNG bitmap under Node.js, execute the following at the command prompt:
+
+```
+tiddlywiki mywiki --screenshot $:/plugins/tiddlywiki/innerwiki/examples
+```
+
+The screenshots will be saved as `screenshot-1.png` etc in the `./output` folder of the wiki.
+
+!! Clipping
+
+A clipping rectangle can be applied to limit the area of the wiki that is displayed. For example:
+
+<<example """<$innerwiki width="1200" height="400" style="width:100%;" clipLeft="500" clipTop="100" clipWidth="600" clipHeight="300" filename="screenshot-2">
+	<$data title="HelloThere" text="! This tiddler is inside a wiki that is inside a wiki"/>
+	<$data title="$:/DefaultTiddlers" text="HelloThere"/>
+</$innerwiki>""">>
+
+!! Transcluding payload tiddlers
+
+This example shows how the `<$data>` widget can be transcluded from other tiddlers (see $:/plugins/tiddlywiki/innerwiki/example-data):
+
+<<example """<$innerwiki width="600" height="400" style="width:100%;" filename="screenshot-3">
+	{{$:/plugins/tiddlywiki/innerwiki/example-data}}
+	<$data title="HelloThere" text="! This tiddler is inside a wiki that is inside a wiki"/>
+	<$data title="$:/DefaultTiddlers" text="HelloThere"/>
+</$innerwiki>""">>
+
+!! Customising the wiki state
+
+By injecting the right payload tiddlers, the innerwiki can be initialised to any desired state. In this example we inject a configuration tiddler to make the "more" page control button visible, and a state tiddler to cause the dropdown to appear:
+
+<<example """<$innerwiki template="$:/plugins/tiddlywiki/innerwiki/template" filename="screenshot-4" width="1200" height="400" clipLeft="500" clipTop="100" clipWidth="600" clipHeight="300" style="width:100%;">
+	<$data title="HelloThere" text="! This tiddler is inside a wiki that is inside a wiki"/>
+	<$data title="$:/DefaultTiddlers" text="HelloThere"/>
+	<$data title="$:/config/PageControlButtons/Visibility/$:/core/ui/Buttons/more-page-actions" text="show"/>
+	<$data title="$:/state/popup/more--1600698846" text="(151,144,21,25)"/>
+</$innerwiki>""">>
+
+!! Inception
+
+An innerwiki can itself contain an inner-innerwiki:
+
+<<example """<$innerwiki width="1200" height="600" style="width:100%;" filename="screenshot-5">
+	<$data title="HelloThere" text="! This tiddler is inside a wiki that is inside a wiki"/>
+	<$data title="$:/DefaultTiddlers" text="HelloThere $:/plugins/tiddlywiki/innerwiki/inner-example"/>
+	<$data $tiddler="$:/plugins/tiddlywiki/innerwiki"/>
+</$innerwiki>""">>
+
+(You can see the innerwiki here: $:/plugins/tiddlywiki/innerwiki/inner-example)
+
+Note the way that the innerwiki plugin has to be explicitly added to the innerwiki.