restyle builtin modules to match project style

Now that the IWYU cleanup has been merged compile all, not just a couple, of
the builtin modules independent of builtin.cpp. That is, no longer `#include
builtin_NAME.cpp` in builtin.cpp. This is more consistent, more in line with
what developers expect, and is likely to reduce mistakes.

Reduces lint errors from 384 to 336 (-13%). Line count from 6307 to 4988 (-21%).

Another step in resolving issue #2902.

src/builtin_printf.h

@@ -0,0 +1,11 @@
+// Prototypes for functions for executing builtin_printf functions.
+#ifndef FISH_BUILTIN_PRINTF_H
+#define FISH_BUILTIN_PRINTF_H
+
+#include <wchar.h>
+#include <cstring>
+
+class parser_t;
+
+int builtin_printf(parser_t &parser, io_streams_t &streams, wchar_t **argv);
+#endif