Improve `generateContentFn` helper scriptlet

New directive: `join:[separator][sub-directives]`

[separator] is an author-defined two-character string to be used to split
the following sub-directives string. The sub-directives are fed back into
the helper scriptlet to generate sub-content, which will be joined into
a single string. Example:

...##+js(trusted-prevent-fetch, propstomatch, join:--length:10-20--[some literal content]--length:80-100-- push['ads'])