Add future property to simpleconfig stub

[dextermb]

Here's a start to adding the future property to the simpleconfig stub. I'm not sure on the best way to put the featureFlags.future into the file, but with some direction I'll gladly implement that too.

Closes #2370

[adamwathan]

Honestly probably best to just put them in manually like this:

future: {
  // removeDeprecatedGapUtilities: true,
  // purgeLayersByDefault: true,
}

[dextermb]

Honestly probably best to just put them in manually like this:

future: {
  // removeDeprecatedGapUtilities: true,
  // purgeLayersByDefault: true,
}

Done. Would be interesting to see if we can automate this process a little bit, but I suppose that'd require some more complex writes to the file during pre-commit or something.