Add a serve field to deno.json to specify the serve entry

[LitoMore]

How about adding a new field serve to deno.json to specify the serve entry?

// deno.json
{
+	"serve": "source/app.ts"
}

After this users should be able to start the service directly by running deno serve.

This is more convenient than a deno task serve. We can also add deno serve optional support in deno.json like deno fmt does.

What do you think?