Patch highlight-selected's HighlightedAreaView to only highlight up t…

…o 400 markers

Summary:
If this is unbounded, `highlightSelectionInEditor` can block significantly. In my tests, highlighting 100k nodes took about 10s.

Since the usefulness of this degrades past a handful of selections, patch it to limit the number of highlights to 400. This results in pauses anywhere from 50s-150s in extreme cases.

Reviewed By: aadisriram

Differential Revision: D13033448

fbshipit-source-id: 675e8fa0b15e8e960adc21e1a58e9aac24c9d265

flow-libs/atom.js.flow

@@ -416,6 +416,7 @@ declare class atom$PackageManager {
   deactivatePackages(): Promise<void>,
   deactivatePackage(name: string, suppressSerialization?: boolean): Promise<void>,
   emitter: atom$Emitter,
+  loadedPackages: {[packageName: string]: atom$Package},
   loadPackage(name: string): void,
   loadPackages(): void,
   serializePackage(pkg: atom$Package): void,