fix(classes): Make document state available during class's post init

Author: alerque

classes/base.lua

@@ -86,6 +86,7 @@ function class:_init (options)
 end
 
 function class:_post_init ()
+   SILE.documentState.documentClass = self
    self._initialized = true
    for i, func in ipairs(self.deferredInit) do
       func(self)

inputters/base.lua

@@ -31,7 +31,8 @@ function inputter:classInit (options)
    if constructor.id then
       SU.deprecated("std.object", "pl.class", "0.13.0", "0.14.0", string.format(_deprecated, constructor.id))
    end
-   SILE.documentState.documentClass = constructor(options)
+   -- Note SILE.documentState.documentClass is set by the instance's own :_post_init()
+   constructor(options)
 end
 
 function inputter:requireClass (tree)