Add autofix suggestion for method calls mistaken as field access

[Wilfred]

Summary

This PR adds an autofix suggestion when a user attempts to access a field that doesn't exist but matches a method name on the type. For example, accessing items.len will now suggest items.len() with an automatic fix.

Key Changes

• Enhanced error diagnostics in type_checker.rs to detect when a field access name matches an available method on the type
• When a mismatch is detected, the error message now includes a helpful suggestion: "Did you mean method_name()?"
• Added an Autofix that automatically converts the field access to a method call (e.g., items.len → items.len())
• Applied this improvement to two error paths in the type checker (struct field access and general field access)
• Added test case list_len_method.gdn demonstrating the autofix in action
• Updated CHANGELOG.md to document the new feature

Implementation Details

• The fix retrieves available methods from the type environment and checks if the accessed name matches any method
• Only generates the suggestion and autofix when a matching method is found
• The autofix includes both a description and the corrected code text for IDE integration

https://claude.ai/code/session_014eWiDph7M3enBMha1hczaQ