Chat: Display all used and ignored items

[abeatrix]

Followup on https://github.com/sourcegraph/cody/pull/4479

Chat: Display all used and ignored items

• Refactor context tracking and display logic in DefaultPrompter
• Update PromptBuilder to consistently set isTooLarge flag
• Adjust ContextCell UI to use more accurate "items" terminology
• Move history items to priority below the new Enhanced context

Test plan

• Run Ollama (so we can use a model with smaller context window)
• Start Cody from this branch
• Start a conversation with Cody and ask Cody questions with enhanced context
• You should start seeing items being excluded as the conversation gets longer