refactor(packages)!: Process bibtex content as XML not SIL

BREAKING CHANGE: The primary use was probably internal to SILE, but if
by chance you have bibtex databases with formatting commands in SIL
markup format rather than just plain text content the markup will cease
to function and will need to be converted to XML syntax instead.

This enables the use of declarative markup in Fluent localizations.

packages/bibtex/bibliography.lua

@@ -381,7 +381,7 @@ Bibliography = {
     italic = function (...)
       local t = {...}
       return function(item)
-        return Bibliography._process(item, t, "\\em{", "}")
+        return Bibliography._process(item, t, "<em>", "</em>")
       end
     end,
     parens = function (...)

packages/bibtex/init.lua

@@ -96,7 +96,7 @@ function package:registerCommands ()
       SU.warn("Unknown reference in citation "..options.key)
       return
     end
-    SILE.processString(cite, "sil")
+    SILE.processString(("<sile>%s</sile>"):format(cite), "xml")
   end)
 
   self:registerCommand("reference", function (options, content)
@@ -112,7 +112,7 @@ function package:registerCommands ()
       SU.warn("Unknown type @"..err.." in citation for reference "..options.key)
       return
     end
-    SILE.processString(cite, "sil")
+    SILE.processString(("<sile>%s</sile>"):format(cite), "xml")
   end)
 
 end