Lua: register script name in global variable

The name of the Lua script which is executed is made available in the
global Lua variable `PANDOC_SCRIPT_FILE`, both for Lua filters and
custom writers.

Closes: #4393

test/lua/script-name.lua

@@ -0,0 +1,3 @@
+function Para (_)
+  return pandoc.Para{pandoc.Str(PANDOC_SCRIPT_FILE)}
+end