more robust JS preprocessor

The patch is a complete rework of JS preprocessing with stricter syntax
validation. It also adds support for more expressive preprocessor
directives like the following:

  #include "foobar.js"
  #if !(VAR1 /* comment */ && VAR2 >= 2)
  #elif !VAR3 || VAR4 != "four" // comment
  #else
  #endif

The directives are parsed according to an LL(1) grammar, using a stack
for syntax-directed translation.

Author: hujiajie