Improve error messages with source location

[dev-dami]

Description

Error messages should show the exact line and column where the error occurred, with a snippet of the source code.

Current Behavior

Error: undefined variable 'foo'

Expected Behavior

Error: undefined variable 'foo'
  --> main.carv:5:10
   |
 5 |     let x = foo + 1;
   |             ^^^

Where to Start

• pkg/eval/eval.go - Errors are generated here
• pkg/types/checker.go - Type errors
• pkg/parser/parser.go - Parse errors
• Token structs already have Line/Column info

Good First Issue

This improves developer experience significantly! Can be done incrementally - start with one error type.