zig fmt is built directly into stage1 rather than child process

Previously, `zig fmt` on the stage1 compiler (which is what we currently
ship) would perform what equates to `zig run std/special/fmt_runner.zig`

Now, `zig fmt` is implemented with the hybrid zig/C++ strategy outlined
by #1964.

This means Zig no longer has to ship some of the stage2 .zig files, and
there is no longer a delay when running `zig fmt` for the first time.

src/userland.cpp

@@ -37,3 +37,8 @@ void stage2_render_ast(struct Stage2Ast *ast, FILE *output_file) {
     const char *msg = "stage0 called stage2_render_ast";
     stage2_panic(msg, strlen(msg));
 }
+
+int stage2_fmt(int argc, char **argv) {
+    const char *msg = "stage0 called stage2_fmt";
+    stage2_panic(msg, strlen(msg));
+}