feat: implement string interpolation

Add support for string interpolation with {expr} syntax:
  "Hello, {name}!" becomes "Hello, " + toString(name) + "!"

Lexer changes:
- Add StringPart enum (Literal/Expr) and InterpolatedString token
- Detect {expr} in strings and capture expression text
- Support escaped braces with \{ and \}

Parser changes:
- Add desugar_interpolated_string() to convert to concatenation
- Automatically wrap expressions in toString() calls

Interpreter changes:
- Fix toString() to not add quotes around strings

Tests added:
- 4 lexer tests for interpolation tokenization
- 4 integration tests for full interpolation pipeline

Add examples/interpolation.lux demonstrating:
- Variable interpolation
- Number interpolation (auto toString)
- Expression interpolation ({a + b})
- Escaped braces

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/interpreter.rs

@@ -1798,7 +1798,13 @@ impl Interpreter {
                 if args.len() != 1 {
                     return Err(err("toString requires 1 argument"));
                 }
-                Ok(EvalResult::Value(Value::String(format!("{}", args[0]))))
+                // For strings, return the string itself (no quotes)
+                // For other values, use Display formatting
+                let result = match &args[0] {
+                    Value::String(s) => s.clone(),
+                    v => format!("{}", v),
+                };
+                Ok(EvalResult::Value(Value::String(result)))
             }
 
             BuiltinFn::TypeOf => {