dillon1000/react

[Events] Add support for events that are both user-blocking and continuous (#15811)

* [Events] Add EventPriority enum

React DOM's DispatchConfig for synthetic events has an `isDiscrete`
field that affects how updates triggered by an event are scheduled.
Events are either discrete or continuous.

This commit adds an additional type of configuration where an event
has user-blocking priority, but is not discrete. E.g. updates triggered
by hover are more important than the default, but they don't need to
be processed serially. Because there are now three types of event
priority instead of two, I've replaced the `isDiscrete` boolean with an
enum: `eventPriority`.

This commit implements the new enum value but does not change any
behavior. I'll enable it behind a feature flag in the next commit.

I've only implemented this in the legacy event system. I'll leave Flare
for a follow-up.

* enableUserBlockingEvents feature flag

Adds a feature flag to increase the priority of events like `mouseover`,
without making them discrete.