runtime(c): add new constexpr keyword to syntax file (C23)

closes: #16471

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/c_keywords.c

@@ -0,0 +1,64 @@
+// C keywords
+
+// Source: https://en.cppreference.com/w/c/keyword
+
+alignas // (C23)
+alignof // (C23)
+auto
+bool // (C23)
+break
+case
+char
+const
+constexpr // (C23)
+continue
+default
+do
+double
+else
+enum
+extern
+false // (C23)
+float
+for
+goto
+if
+inline // (C99)
+int
+long
+nullptr // (C23)
+register
+restrict // (C99)
+return
+short
+signed
+sizeof
+static
+static_assert // (C23)
+struct
+switch
+thread_local // (C23)
+true // (C23)
+typedef
+typeof // (C23)
+typeof_unqual // (C23)
+union
+unsigned
+void
+volatile
+while
+
+_Alignas // (C11)(deprecated in C23)
+_Alignof // (C11)(deprecated in C23)
+_Atomic // (C11)
+_BitInt // (C23)
+_Bool // (C99)(deprecated in C23)
+_Complex // (C99)
+_Decimal128 // (C23)
+_Decimal32 // (C23)
+_Decimal64 // (C23)
+_Generic // (C11)
+_Imaginary // (C99)
+_Noreturn // (C11)(deprecated in C23)
+_Static_assert // (C11)(deprecated in C23)
+_Thread_local // (C11)(deprecated in C23)