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.

gtests/TestFixture.cpp

@@ -60,7 +60,6 @@ EShLanguage GetShaderStage(const std::string& stage)
         return EShLangFragment;
     } else if (stage == "comp") {
         return EShLangCompute;
-#ifdef NV_EXTENSIONS
     } else if (stage == "rgen") {
         return EShLangRayGenNV;
     } else if (stage == "rint") {
@@ -77,7 +76,6 @@ EShLanguage GetShaderStage(const std::string& stage)
         return EShLangTaskNV;
     } else if (stage == "mesh") {
         return EShLangMeshNV;
-#endif
     } else {
         assert(0 && "Unknown shader stage");
         return EShLangCount;