[oss] Add feature flag for new asset selection syntax (#25999)

as titled.

Not actually exposing the flag to users just yet, but do want to gate
the code with it so landing the flag and will enable manually with JS.

js_modules/dagster-ui/packages/ui-core/src/app/FeatureFlags.oss.tsx

@@ -4,6 +4,7 @@ export enum FeatureFlag {
   flagSidebarResources = 'flagSidebarResources',
   flagDisableAutoLoadDefaults = 'flagDisableAutoLoadDefaults',
   flagLegacyRunsPage = 'flagLegacyRunsPage',
+  flagAssetSelectionSyntax = 'flagAssetSelectionSyntax',
 
   // Flags for tests
   __TestFlagDefaultNone = '__TestFlagDefaultNone',