Allow attribute substitution to handle variables containing non-word characters (#7606)

* Added failing test for #7604

* Fix attribute substitution regexp

Use the same regexp in wiki.getSubstitutedText as is used in
Widget.prototype.substituteVariableReferences. Fixes #7604.

* Added a test for a variable name containing spaces

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

@@ -6,14 +6,16 @@ tags: [[$:/tags/wiki-test-spec]]
 title: Output
 
 \whitespace trim
-<$let project="TiddlyWiki" disabled="true">
+<$set name="var with spaces" value="spaces">
+<$let project="TiddlyWiki" disabled="true" var-with-dashes="dashes">
 <div class=`$(project)$ 
-${ [[Hello]addsuffix[There]] }$` attrib=`myvalue` otherattrib=`$(1)$` blankattrib=`` quoted="here" disabled=```$(disabled)$```>
+${ [[Hello]addsuffix[There]] }$` attrib=`myvalue` otherattrib=`$(1)$` blankattrib=`` quoted="here" disabled=```$(disabled)$``` dashes=`$(var-with-dashes)$` spaces=`$(var with spaces)$`>
 </div>
 </$let>
+</$set>
 
 +
 title: ExpectedResult
 
 <p><div attrib="myvalue" blankattrib="" class="TiddlyWiki 
-HelloThere" disabled="true" otherattrib="" quoted="here"></div></p>
+HelloThere" dashes="dashes" disabled="true" otherattrib="" quoted="here" spaces="spaces"></div></p>