Disallow `try` and `catch` used together

[mjm918]

Summary

Using try and catch together on the same expression is semantically redundant and should emit a compiler error.

Current Behavior

The following code compiles but is meaningless:

var result: int = try safe_divide(10, 0) catch e {
    println("Division by zero caught!");
};

Expected Behavior

The compiler should emit an error:

error: `try` and `catch` cannot be used together
  --> file.naml:10:5
   |
10 |     var x = try safe_divide(10, 0) catch e { ... };
   |             ^^^ remove `try` - `catch` already handles the exception
   |
help: use either `try expr ?? default` or `expr catch e { handler }`

Valid Forms

// Form 1: Try with default (converts to option, provides fallback)
var x: int = try divide(10, 0) ?? -1;

// Form 2: Catch with handler block (for side effects/early return)
var x: int = divide(10, 0) catch e {
    println(e.message);
    return;
} ?? -1;

Rationale

• try expr converts a throwing expression to option<T>
• expr catch e { handler } handles the exception with a block
• Combining them adds no semantic value - catch already handles the exception

Implementation

Add validation in the parser or type checker to detect when try wraps an expression that also has a catch block, and emit an appropriate error.