purescript-indentation.el: improve compiler visibility into local variables

[Hi-Angel]

Turning on lexical-binding fixes a bunch of warnings like:

purescript-indentation.el:399:47: Warning: ‘pi’ is an obsolete variable (as of 23.3); use ‘float-pi’ instead.

these warnings are caused by Emacs not being able to determine that pi is actually a local variable. By making use of lexical binding we make Emacs to be able to see through that.