[6s] Break out builtin commands into a list of structs

The 6s builtin commands are now in a separate file, with a list of name,
description, and function pointers.

src/user/6s/commands.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <stddef.h>
+
+namespace edit {
+    class source;
+}
+
+
+class builtin
+{
+public:
+    using runf = void (*)(edit::source &);
+
+    builtin(const char *name, const char *desc, runf func) :
+        m_name {name}, m_desc {desc}, m_func {func} {}
+
+    const char * name() const { return m_name; }
+    const char * description() const { return m_desc; }
+
+    void run(edit::source &s) { m_func(s); }
+
+private:
+    const char *m_name;
+    const char *m_desc;
+    runf m_func;
+};
+
+
+extern builtin g_builtins[];
+extern size_t  g_builtins_count;