Describe lexical binding before dynamic

Describe lexical binding before dynamic

* doc/lispref/variables.texi (Variable Scoping)
(Lexical Binding, Dynamic Binding):
Alter the presentation order from the point of view that lexical
binding is the standard discipline (if not always the default) and
dynamic binding an alternative, which corresponds better to Elisp
today.  Modernise parts of the text.
* doc/lispref/elisp.texi (Top): Update menu.