fix(packages): Option "plural" on bibliography terms can be false

Some styles explicitly set plural=false (though this is the default
value) and we were missing a boolean conversion, so we always
assumed true in that case...

Author: Omikhleia

packages/bibtex/csl/engine.lua

@@ -482,7 +482,7 @@ function CslEngine:_text (options, content, entry)
          SU.error("CSL macro " .. options.macro .. " not found")
       end
    elseif options.term then
-      t = self:_render_term(options.term, options.form, options.plural)
+      t = self:_render_term(options.term, options.form, SU.boolean(options.plural, false))
    elseif options.variable then
       variable = options.variable
       t = entry[variable]