feat(chapi-ast-c): add support for additional macro keyword in C.g4 #24

Add support for the macro keyword 'ifdef' in the 'conditionalDeclaration' rule of the C.g4 file in chapi-ast-c/src/main/antlr. This allows for more comprehensive parsing of C code.

chapi-ast-c/src/main/antlr/C.g4

@@ -40,10 +40,14 @@ compilationUnit
 oneLineMacroDeclaration
     : '#' include (StringLiteral | ('<' includeIdentifier '>' ))         #includeDeclaration
     | '#' 'define' expression*                                           #defineDeclaration
-    | '#'( 'if' | 'undef' | 'else' | 'pragma' | 'endif' )  expression*   #conditionalDeclaration
+    | '#' macroKeywords  expression*   #conditionalDeclaration
     | Identifier postixCall?  compoundStatement?                         #macroCall
     ;
 
+macroKeywords
+    :  'if' | 'undef' | 'else' | 'pragma' | 'endif' | 'ifdef'
+    ;
+
 MultiLineMacro
     : '#' (~[\n]*? '\\' '\r'? '\n')+ ~ [\n]+ -> channel (HIDDEN)
     ;