Add migration script for mirroring

[foolip]

No description provided.

[foolip]

I haven't written any tests for this, because no matter the amount of tests I think they have no value. The proof that this is good is by comparing build/data.json before and after using it.

[foolip]

This is able to replace ~75k support statements across 2000+ files, but I think the way to use this should be:

• First node scripts/migrations/009-mirror.js --browsers chrome_android edge firefox_android opera opera_android safari_ios samsunginternet_android webview_android for the cases that can be mirrored in all browsers, no complications to look into
• Then run things like node scripts/migrations/009-mirror.js --browsers chrome_android webview_android and node scripts/migrations/009-mirror.js opera opera_android and for smaller groups of things that will often be mirror-able
• Finally go through mirroring the individual browsers. This is the step where a lot of odd discrepancies might be discovered.

[queengooborg]

Could we set this to migration 009? I have a pull request for a migration 008 that's waiting on your review!

[foolip]

I've renamed to 009. But also, do we need the numbers? We're missing some in the series, and will also delete scripts that have served their purpose.

[queengooborg]

Some migrations' scripts were deleted because they were just runs of npm run fix. But you're right; the numbers aren't too useful when compared to the review and merge timing. Let's opt to remove them in a future PR.

[foolip]

@queengooborg this is ready for review now.

[queengooborg]

I haven't tried running the script, but this looks great to me -- let's start migrating browsers to mirror!