upgradeability: include inherited private variables, ignore immutables

[0xalpharush]

The upgradeability check doesn't consider inherited, private variable declarations nor does it consider immutability. This PR addresses that by using state_variables_ordered and checking that variables aren't immutable prior to flagging them.

// test.sol 
pragma solidity ^0.8.0;
contract A {
    uint private a = 1; // not currently detected 
}
contract B is A {
    uint a = 2;
    uint immutable b = 3; // fine to delegatecall (same as constant)
}

Command: slither-check-upgradeability test.sol B
Before:

B.a (test.sol#6) is a state variable with an initial value.
B.b (test.sol#7) is a state variable with an initial value.

After fix:

A.a (test.sol#3) is a state variable with an initial value.
B.a (test.sol#6) is a state variable with an initial value.

[0xalpharush]

Probably needs to be done here too

slither/slither/tools/upgradeability/checks/variables_order.py

Line 112 in b96beea

order1 = [variable for variable in contract1.state_variables if not variable.is_constant]

[montyly]

@0xalpharush: can you make the changes in the other places within this PR?