[Proposal] Introduce `Dim` tuple syntax, discards, and inline `Dim` for out variables

## Summary
VB.NET does not yet support `Dim` tuple deconstruction syntax, despite consistent community requests dating back to 2018 (see: [dotnet/vblang#346](https://github.com/dotnet/vblang/issues/346)). As of 2026, modern coding tools and third-party extensions have begun emulating this syntax, highlighting strong demand for first-class support.

This feature would bring VB.NET parity with modern tuple patterns, improve code brevity, and simplify common patterns like deconstruction, inline out variables, and value discards.

## Proposed Syntax & Examples

### 1. Tuple Deconstruction with `Dim` (Type Inference & Explicit Typing)
Support direct tuple deconstruction using the `Dim` statement, with both inferred and explicit type support:
```vb
' Dim tuple syntax with local type inference
Dim (a, b) = (3, 5)

' Explicit type annotation for tuple elements
Dim (a As Integer, b As Integer) = (3, 5)
```

### 2. Discard Slots (Empty Deconstruction Slots)
Allow empty slots in tuple deconstruction to **discard unused values** (a common and useful pattern in modern .NET languages):
```vb
' Discard the first value; assign only the second and third elements
Dim (, a, b) = (3, 5, 7)
' Result: a = 5, b = 7; first value (3) is ignored
```

### 3. Inline `Dim` for C#'s `out` Variables
Simplify the usage of C#'s `out` parameters in VB.NET with **inline `Dim` syntax** (automatic type inference, no pre-declaration required):
```vb
Dim input = Console.ReadLine()

' Inline Dim for out variable - type is inferred automatically
If Integer.TryParse(input, Dim x) Then
' x is strongly typed as Integer
Dim y = 3 * x - 2
Console.WriteLine("x = {0}, y = {1}", x, y)
End If

' Discard the out variable entirely using an empty slot
If Integer.TryParse(input, ) Then Console.WriteLine("Input is a valid integer.")
```

## Motivation & Benefits
1. **Modernize VB.NET**: Align with contemporary .NET language patterns for tuples and discards.
2. **Reduce Boilerplate**: Eliminate redundant variable declarations for `out` parameters and tuple unpacking.
3. **Community Demand**: Long-requested feature with widespread support from VB.NET developers.
4. **Tooling Consistency**: Match the behavior already being emulated by popular coding tools.

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In conclusion, as both a C# and VB.NET developer, I strongly encourage the VB.NET language team to prioritize implementing this critical productivity feature. Many thanks for your consideration!