Simplify string literal completions

[ghost]

Instead of creating CompletionInfo directly, return a union of several kinds and convert to CompletionInfo at the end. This is useful because we won't always want a CompletionInfo (#21374). It also cuts down on code since we were duplicating the code to create CompletionInfo in a lot of places.

Note: Previously, getStringLiteralCompletionEntriesFromCallExpression returned isNewIdentifierLocation: true, but isNewIdentifierLocation: false returned isNewIdentifierLocation: false. In both cases, we are getting the string literal from a contextual type, so I would expect these situations to work the same, and now we return isNewIdentifierLocation: true in both locations. Relevant test completionForStringLiteral3 which tests for this was added in #8428. @mhegazy