Chat: Include editor selection in chat context by default

[abeatrix]

Summary

This change includes the user's current editor selection as context for all new chat messages by default. The selection context is added to the user context items when creating a new chat message.

To support this, the getContextFileFromSelection function is now imported and used in SimpleChatPanelProvider.ts. If the chat message source is 'chat', the current selection context is retrieved and added to the userContextFiles array before resolving the context items.

The selection context retrieval logic is also removed from getPriorityContext in context.ts since it is now handled in SimpleChatPanelProvider.ts.

Test plan

1. Build Cody from this branch
2. Highlight some code in your editor
3. Start a new chat and ask Cody about the code you highlighted
4. Submit the question with Opt + Enter (enhanced context disabled)
5. Confirm your selection is included as context, and Cody is able to answer your question
6. Start a new empty chat and ask Cody about the code you highlighted
7. Submit the question with Enter (enhanced context enabled)
8. Confirm your selection is also included as context with enhanced context added, and Cody is able to answer your question

My highlighted code

Enhanced Context disabled:

Enhanced Context enabled:

[jtibshirani]

It makes sense to me to always include the selection, and count it towards the "user-supplied context" budget.

This change looks good to me, but someone from Cody should review too :)

CC @chillatom since you're leading the charge on the enhanced context changes for the next release!