feat: add deno x (alias dx) for conveniently running binaries from packages

[nathanwhit]

Open questions:

• Default permissions?
  • Right now it defaults to no permissions, but when you set up the dx alias, you can pass --default-allow-all to opt in to allow all unless otherwise specified. So with that opt-in dx rolldown would require no other flags, and run with allow all, but dx -R rolldown would only get read permissions
  • Could just reject the idea, and so you would have to do dx -A rolldown to get --allow-all
• Allow scripts when installing the package?
  • pnpx, for instance, runs scripts and doesn't require approve-builds
  • If we do run them by default, it's sort of the equivalent of allow-all by default
  • If we don't run them or make you pass a flag, then a lot of binaries won't work correctly
• npm: by default?
  • currently, defaults to npm: unless otherwise specified
  • discussed: yes
• what if user passes in a file path or other specifier? e.g. dx main.ts
  • Error out? Treat it like deno run? Do what npx and other package managers do and run the npm package called main.ts (because of course that's a thing)
  • Discussed: if file, error