use the accurate docContext and persist repo visibility using local storage

[hitesh-1997]

The PR improves the following:

1. When calculating the diff over time for suggestion, we were using document.getText. Analyzing offline telemetry we see in ~20% we get some extra chars if user types between calculating docContext and loaded call. We leverage already computed completePrefix and completeSuffix to compute the text, which is accurate.
2. We were using in-memory cache for calculating repoVisibility before. But without passing token, we frequently hit rate limit for querying github api and cached tokens are evicted from in-memory cache when user refresh. This PR stores the tokens in localStorage instead of in-memory store to cache the repo visibility.

Test plan

Manual Testing

1. Adding debugger and refreshing to see if the token value is persisted/ deleted from local storage.