refactor(cs): Refactor and optimize active stream checking

Description

This MR refactors and optimizes how we check for whether to go down the streaming branch or default branch.

• Change: If there's an active stream at the current position, we do not need to check for intention.
• Change: If we are going down the default branch, let's go ahead and cancel out all other streams.
• Refactor: Move things over to privates for better encapsulation
• Refactor: Add provideDefaultInlineCompletionItems private method so that we are not creating a closure for this on every keystroke.

Screenshots

With streamCodeGenerations FF on	With streamCodeGenerations FF off	Code completion still works