Add --dangerous-sourcemap option to suppress warning

[kentcdodds]

If you follow sentry's remix guide it'll tell you to enable the --sourcemap option when building, then upload the sourcemap, then delete the sourcemap files.

When you use the --sourcemap flag, you get this warning:

 warn  🚨  source maps enabled in production
┃ You are using `--sourcemap` to enable source maps in production,
┃ making your server-side code publicly visible in the browser.
┃ This is highly discouraged!
┃ If you insist, ensure that you are using environment variables for secrets
┃ and are not hard-coding them in your source.
┗

But this isn't a problem if you're deleting the sourcemaps anyway.

What would be best is if there were some way to have client-side sourcemaps that somehow didn't reveal server-side code, but until we get that, it would be at least be nice to suppress this warning.

I propose a flag called --dangerous-sourcemap.