Add textEditor tool that combines readFile and updateFile functionality

[bhouston]

This PR implements GitHub Issue #59 by creating a new textEditor tool that combines the functionality of readFile and updateFile into a single tool, similar to Anthropic's built-in text editor tool. The new tool provides the following commands:

• view: Display file content with line numbers or directory listing
• create: Create a new file with specified content
• str_replace: Replace specific text in a file (must be unique)
• insert: Insert text at a specific line number
• undo_edit: Revert the last edit made to a file

The tool maintains state across calls, allowing for undo functionality. All tests pass and the tool is fully integrated into the agent.