Adds Text substitution support in widget attributes and new operator (#7526)

* feat: new text substitution support, first pass

* fix: use the widget method instead of evaluating a filter

* revert to earlier implementation that emulates macro syntax

* fix: capitalize comments

* feat: add support for triple backticks for substituted attributes

* docs: added docs for substitute operator

* chore: more docs tweaks

* docs: substituted attributes, refactored docs for widget attributes

* docs: fixed typo

* docs: more examples for substituted attributes

* docs: updated prior documentation on concatenating text and variables

* docs: documentation corrections

* Update editions/tw5.com/tiddlers/filters/examples/substitute Operator (Examples).tid

Co-authored-by: btheado <brian.theado@gmail.com>

---------

Co-authored-by: btheado <brian.theado@gmail.com>

editions/test/tiddlers/tests/data/widgets/SubstitutedAttributes.tid

@@ -0,0 +1,19 @@
+title: Widgets/SubstitutedAttributes
+description: Attributes specified as string that should have substitution performed.
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\whitespace trim
+<$let project="TiddlyWiki" disabled="true">
+<div class=`$(project)$ 
+${ [[Hello]addsuffix[There]] }$` attrib=`myvalue` otherattrib=`$(1)$` blankattrib=`` quoted="here" disabled=```$(disabled)$```>
+</div>
+</$let>
+
++
+title: ExpectedResult
+
+<p><div attrib="myvalue" blankattrib="" class="TiddlyWiki 
+HelloThere" disabled="true" otherattrib="" quoted="here"></div></p>