[0.84] Types: Annotate Promise<void> returns on async functions

[robhogan]

Summary:
flow-api-translator assumes that a function without an explicit return type
annotation returns void. That isn't true of async functions, which always
return a Promise, so our generated .d.ts declares seven async functions as
returning void - including the public Server#end(), Watcher#watch(),
Watcher#close() and DependencyGraph#end(). TypeScript consumers awaiting
those get await-thenable from typescript-eslint, or quietly don't await at
all.

I've fixed the translator upstream in
facebook/flow#9486, but 0.84.x pins
flow-api-translator 0.35.0 and I'd rather not bump the toolchain on a release
branch, so this annotates the return types at source instead. It's a no-op for
Flow, which already infers Promise<void> in every case here, and it's what we
want in the source regardless. main will pick the translator fix up with the
next version bump, after which these annotations are still correct, just no
longer load-bearing.

To find the full set I translated every file the generator covers with 0.35.0
patched and unpatched and diffed the output - these seven declarations, across
five files, are all of them on this branch.

Changelog:

 - **[Fix]**: Types: async methods including `Server#end`, `Watcher#watch`/`#close` and `DependencyGraph#end` are declared as returning `Promise<void>` rather than `void`

Test plan:

yarn run build-ts-defs   # updates exactly the five .d.ts files, no other churn
yarn typecheck           # No errors!
yarn typecheck-ts
yarn jest packages/metro/src/Server/__tests__/Server-test.js packages/metro-file-map/src/__tests__ packages/metro-file-map/src/watchers/__tests__