New rules for checking that no non-initialized immutables are used in the declaration of other immutables

[PaulRBerg]

It'd be helpful to provide a new Solhint rule for checking that no non-initialized immutable variables are used in the declaration of other immutable variables.

Note that 'non-initialized' means that the immutable Foo is declared AFTER the other immutable Bar uses Foo.

It's clearer to see the issue with this code:

contract Immutables {
    // CASE 1: Constants can be referenced before declaration
    // constA correctly uses constB even though constB is declared later
    uint256 public constA = constB + 100;     // Will be 150
    uint256 internal constant constB = 50;    // Constants are compile-time values

    // CASE 2: Immutables CANNOT be used before declaration
    // immA tries to use immB before immB is declared - this causes a problem!
    uint256 public immA = immB + 100;         // Will be 100, NOT 125!
    uint256 internal immutable immB = 25;     // immB isn't available when immA is initialized

    // CASE 3: Correct order for immutables
    // Declare immutables first, then use them
    uint256 internal immutable immD = 75;     // Declare immutable first
    uint256 public immC = immD + 100;         // Will be 175 as expected

    // For clarity, constructor shows the actual values
    constructor() {
        // Runtime values demonstration
        assert(constA == 150);    // Constants always work regardless of order
        assert(immC == 175);      // Correct usage of immutables

        // Uncomment this assertion and see the revert
        // assert(immA == 125);
    }
}