Introduce let filter run prefix for assigning a filter run result to a variable

[Jermolene]

This PR introduces a new filter run prefix :let that assigns the result of the filter run to a variable that is made available for the remaining filter runs of the filter expression.

It solves the problem that otherwise it is impossible to compute values for filter operator parameters; parameters can only be a literal string, text reference or variable reference. The :let filter run prefix provides a solution:

:let:index[my.custom.function<param>,[2]] [<data>jsonget[top],<index>]

Note that within the filter run the all[] operator (with a blank parameter) will return all the input titles received by the :let filter run prefix, not the input titles received by the overall filter expression. This permits the results of other filter run prefixes to be assigned to a variable. For following example first sorts all the input tiddlers by length, and then uses the length of the shortest title as the second index of a JSON lookup:

[all[tiddlers]] :sort:number[get[text]length[]] :let:index[all[]] [<data>jsonget[top],<index>]

This PR also introduces the ability for variables to contain list of results, not just single strings. They can be assigned with the :let filter run prefix, or the new <$letlist> widget. The full list of values can be retrieved with the new varlist operator; in other contexts only the first item in the list is used as the variable value.

Note that this PR is intended to be a more generic replacement for the experiment :apply filter operator that is currently part of #8702 - see the code and the docs

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[pmario]

I do get an RSOD if I do :let:test in the AdvancedSearch Filter tab.

Show screenshot

[pmario]

I am not overly happy with special handling of a :let:test[all[]] -- all[] usually means all tiddlers. Now it means only the first one. IMO that's inconsistent and confusing.

May be

• all[first] -- a b :let:index[all[first]] [<data>jsonget[top],<index>]

IMO first would also make sense in general, so we could remove the "special" handling?!

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I did play with: [tag[HelloThere]format:titlelist[]join[ ]] :let:test[all[]] [enlist<test>] to test several elements. It's complex

I could imagine an all[prerun] which would give us a much shorter version of the above test filter [tag[HelloThere]] :let:test[all[prerun]] [<test>enlist-input[]]

[pmario]

The RSOD only triggers if the input is typed. Not if copy / pasted. :letabc also triggers an RSOD

[Jermolene]

I do get an RSOD if I do :let:test in the AdvancedSearch Filter tab.

Thanks @pmario that should be fixed now.

I am not overly happy with special handling of a :let:test[all[]] -- all[] usually means all tiddlers. Now it means only the first one. IMO that's inconsistent and confusing.

I've extended the implementation of variables so that they can store a result list as well as a single value. This allows the let filter run prefix to assign the entire results to the variable. The new varlist[name] operator retrieves a variable as a result list.