chore: add eventTimeline feature flag (#8159)

https://linear.app/unleash/issue/2-2658/create-eventtimeline-feature-flag

Adds a new `eventTimeline` feature flag for the new event timeline
feature.

I think `eventTimeline` is an appropriate name given the feature
description and the way it is evolving, but I'm open to suggestions.
~~This also assumes that this feature will target OSS.~~ Confirmed that
this will be a premium feature.

frontend/src/interfaces/uiConfig.ts

@@ -90,6 +90,7 @@ export type UiFlags = {
     archiveProjects?: boolean;
     projectListImprovements?: boolean;
     onboardingUI?: boolean;
+    eventTimeline?: boolean;
 };
 
 export interface IVersionInfo {

src/lib/types/experimental.ts

@@ -63,7 +63,8 @@ export type IFlagKey =
     | 'addonUsageMetrics'
     | 'onboardingMetrics'
     | 'onboardingUI'
-    | 'projectRoleAssignment';
+    | 'projectRoleAssignment'
+    | 'eventTimeline';
 
 export type IFlags = Partial<{ [key in IFlagKey]: boolean | Variant }>;
 
@@ -312,6 +313,10 @@ const flags: IFlags = {
         process.env.UNLEASH_EXPERIMENTAL_PROJECT_ROLE_ASSIGNMENT,
         false,
     ),
+    eventTimeline: parseEnvVarBoolean(
+        process.env.UNLEASH_EXPERIMENTAL_EVENT_TIMELINE,
+        false,
+    ),
 };
 
 export const defaultExperimentalOptions: IExperimentalOptions = {