Conditional Shortcut Syntax (#7710)

* Initial Commit

* Update docs

* Add support for elseif blocks

* Another test

* WIP

* Change from `{%if%}` to `<%if%>`

See discussion here - https://talk.tiddlywiki.org/t/proposed-if-widget/7882/64

* Don't use the widget body as the template if a list-empty widget is present

See discussion here - https://github.com/Jermolene/TiddlyWiki5/pull/7710#issuecomment-1717193296

* List widget should search recursively for list-template and list-empty

* Allow block mode content within an if/then/else clause

* Update docs

* Add from-version tag to docs

editions/test/tiddlers/tests/data/conditionals/NestedElseif.tid

@@ -0,0 +1,60 @@
+title: Conditionals/NestedElseif
+description: Nested elseif conditional shortcut syntax
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Text
+
+\whitespace trim
+This is a 
+	<% if [<something>match[one]] %>
+		<% if [<another>match[one]] %>
+			Indian
+		<% elseif [<another>match[two]] %>
+			African
+		<% else %>
+			Unknown
+		<% endif %>
+		&#32;Elephant
+	<% elseif [<something>match[two]] %>
+		Antelope
+	<% else %>
+		Crocodile
+	<% endif %>
+, I think.
++
+title: Output
+
+<$let something="one" another="one">
+{{Text}}
+</$let>
+
+<$let something="one" another="two">
+{{Text}}
+</$let>
+
+<$let something="one" another="three">
+{{Text}}
+</$let>
+
+<$let something="two">
+{{Text}}
+</$let>
+
+<$let something="three">
+{{Text}}
+</$let>
++
+title: ExpectedResult
+
+<p>
+This is a Indian Elephant, I think.
+</p><p>
+This is a African Elephant, I think.
+</p><p>
+This is a Unknown Elephant, I think.
+</p><p>
+This is a Antelope, I think.
+</p><p>
+This is a Crocodile, I think.
+</p>