Add #elifdef and #elifndef to preprocessor

[kphillisjr]

A new feature for the preprocessor to reduce complexity for developers familiar with C++23 and C23 in implementing shaders. An example for C++ can be found on the C++ Reference website( See: https://en.cppreference.com/w/cpp/preprocessor/conditional ). This is mostly adding alternative syntax for elements already supported by the language.

Add #elifdef preprocessor macro

This is simply a productivity shortcut similar to #elif where the following statement gets rewritten from:

// Example 1:
#elif defined EXAMPLE_DEFINE_1
// Alternate Syntax (Includes parenthesis) 
#elif defined(EXAMPLE_DEFINE_1)

to:

// Example 1:
#elifdef EXAMPLE_DEFINE_1
// Alternate Syntax (Includes parenthesis) 
#elifdef(EXAMPLE_DEFINE_1)

Add #elifndef preprocessor macro

This is simply an alternate syntax for the following #elif statement:

#elif !defined EXAMPLE_DEFINE
// Alternate Syntax (Includes parenthesis) 
#elif !defined(EXAMPLE_DEFINE)

to:

#elifndef EXAMPLE_DEFINE
// Alternate Syntax (Includes parenthesis) 
#elifndef(EXAMPLE_DEFINE)