[C] Add support for alignof and its aliases (#4075)

This was first added in C11 but has since been revised by C23.
It functions like sizeof but returns alignment instead of size.
So it is implemented as an extension of sizeof.

C++/C.sublime-syntax

@@ -48,7 +48,7 @@ variables:
   type_qualifier: 'const|volatile'
   compiler_directive: 'inline|restrict|__restrict__|__restrict'
   modifiers: '{{storage_classes}}|{{type_qualifier}}|{{compiler_directive}}'
-  non_func_keywords: 'if|for|switch|while|decltype|typeof|typeof_unqual|sizeof|static_assert|_Static_assert|__declspec|__attribute__'
+  non_func_keywords: 'if|for|switch|while|decltype|typeof|typeof_unqual|sizeof|alignof|_Alignof|static_assert|_Static_assert|__declspec|__attribute__'
 
 contexts:
   main:
@@ -197,7 +197,7 @@ contexts:
       scope: keyword.control.flow.return.c
     - match: \b({{control_keywords}})\b
       scope: keyword.control.c
-    - match: \b(sizeof|static_assert|_Static_assert)\b
+    - match: \b(sizeof|alignof|_Alignof|static_assert|_Static_assert)\b
       scope: keyword.operator.word.c
 
   modifiers:

C++/syntax_test_c.c

@@ -224,6 +224,12 @@ typeof_unqual(i) dt;
 /*           ^ punctuation.section.group.begin */
 /*             ^ punctuation.section.group.end */
 
+static_assert(alignof(int) == 4);
+/*              ^ keyword.operator.word */
+
+_Static_assert(_Alignof(int) == 4);
+/*              ^ keyword.operator.word */
+
 static_assert(sizeof(int) == 4);
 /* <- keyword.operator.word */