Web: Remove/rationalize a set of *_EXTENSIONS, using GLSLANG_WEB.

Web: Remove/rationalize a set of *_EXTENSIONS, using GLSLANG_WEB.

Focus was on the front end (not SPIR-V), minus the grammar.
Reduces #ifdef count by around 320 and makes the web build 270K smaller,
which is about 90% the  target size.

The grammar and scanner will be another step, as will the SPIR-V backend.
This makes heavy use of methods #ifdef'd to return false as a global way
of turning off code, relying on C++ DCE to do the rest.