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.

glslang/MachineIndependent/Scan.cpp

@@ -1645,7 +1645,7 @@ int TScanContext::identifierOrType()
         if (const TVariable* variable = parserToken->sType.lex.symbol->getAsVariable()) {
             if (variable->isUserType() &&
                 // treat redeclaration of forward-declared buffer/uniform reference as an identifier
-                !(variable->getType().getBasicType() == EbtReference && afterBuffer)) {
+                !(variable->getType().isReference() && afterBuffer)) {
                 afterType = true;
 
                 return TYPE_NAME;