Default lexical-binding to t

[Stefan Monnier]

In bug#74145 I suggest that it is time to change the default of
`lexical-binding` to t.  Clearly this is a breaking change, but only for
those files which:

- Don't have a `lexical-binding` cookie.
- Have code which happens to behave differently under the new dialect
  (such code is not rare, but a lot of code works identically in the
  two dialects).

I believe by the time Emacs-31 will be released, such files will be
uncommon, and it is easy to fix them (either by adjusting he code, or
by slapping a `lexical-binding` cookie).


        Stefan