[C] Add support for nullptr keyword

It was added in C23 and is effectively an alias for the NULL macro
already in use, so it is just treated as an alias for the existing
null constant.

C++/C.sublime-syntax

@@ -215,7 +215,7 @@ contexts:
       scope: constant.language.boolean.false.c
     - match: \b(true|TRUE)\b
       scope: constant.language.boolean.true.c
-    - match: \bNULL\b
+    - match: \b(nullptr|NULL)\b
       scope: constant.language.null.c
     - match: \b__func__\b
       scope: constant.language.c

C++/syntax_test_c.c

@@ -295,6 +295,12 @@ bool still_C_code_here = true;
 /* <- storage.type */
 /*                       ^ constant.language */
 
+void *null_pointer1 = NULL;
+                    /* ^ constant.language.null */
+
+void *null_pointer2 = nullptr;
+                    /* ^ constant.language.null */
+
 FOOBAR
 hello() {
     /* <- meta.function entity.name.function */