Add feature flags to ParcelOptions

[marcins]

↪️ Pull Request

This PR is the first part of implementing #9539. It adds a strongly typed FeatureFlags to ParcelOptions, which are also available in PluginOptions.

Initially, this change also implemented the ability to set feature flags via .parcelrc, but this was removed as it was a) deemed initially unnecessary if CLI options are available, b) a philosophical shift in what .parcelrc is for, so now it's a lot smaller.

This change has an exampleFeature feature flag that doesn't get used in the code, but is mostly there to differentiate from an empty feature flags object, and to allow for some integration testing.

In the integration tests, it does appear that the RequestGraph correctly gets invalidated by feature flag changes because of invalidateOnOptionChange - however I'm not sure if that's only applying to transformations or other phases as well, I will validate this and follow up if required to ensure feature flags correctly invalidate other parts as well. Admittedly this is an area where I don't have a deep understanding.

A follow-up change will add support for setting feature flag values via CLI options.

💻 Examples

With trunk based development, feature flags will be used to merge changes to v2 that are not intended / ready for most people to use, but we want to test internally. This is as opposed to what might happen today which is a long running branch, and eventually merging when either ready, or behind some unstable options. See #9539 for more details.

🚨 Test instructions

There is an integration test.

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[mattcompiles]

Nice one bud