Improve compiler error messages - hide internal Cranelift errors

[mjm918]

Description

When compilation fails due to internal type mismatches, the error message exposes Cranelift internals:

declared type of variable var0 doesn't match type of value v0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This is confusing for users - they see Rust-specific terms like var0, v0, and RUST_BACKTRACE.

Expected Behavior

Error messages should be user-friendly and naml-specific:

• Show the actual variable name from the source code
• Show the expected vs actual types in naml terms
• Provide a helpful suggestion if possible
• Hide internal implementation details (Cranelift, Rust panic messages)

Example

Current:

declared type of variable var0 doesn't match type of value v0

Expected:

error: type mismatch for variable 'xx'
  --> file.naml:1:17
   |
 1 | var xx: decimal = 1.0;
   |         ^^^^^^^ expected decimal, found float
   |
help: decimal is not yet fully supported, use float instead

Scope

1. Wrap Cranelift compilation in proper error handling
2. Catch panics and convert to user-friendly errors
3. Map internal variable names back to source names where possible
4. Add span information to codegen errors for better source locations