Add support for shift operators for UDVT

## Abstract

Solidity 0.8.19 introduced operator overloading for UDVT. However the shift operator cannot currently be overriden.

```
error[4403]: ParserError: Not a user-definable operator: >>. Only the following operators can be user-defined: |, &, ^, ~, +, -, *, /, %, ==, !=, <, >, <=, >=
```

## Specification

Allow syntax such as

```solidity
using { lshift as <<, rshift as >> } for X global;
```

with requirement that these function have type
```solidity
function lshift(X, uint256) pure returns (X); // use uint8 instead of uint256?
function rshift(X, uint256) pure returns (X); // use uint8 instead of uint256?
```

## Backwards Compatibility

No issue

The issue names no implementation files or tests. Start by tracing parser diagnostic 4403 and the existing user-defined value type operator-overloading path; done means accepting << and >> in the shown using declaration, validating their function signatures, and covering the behavior with tests.

Written by the indexing model from the issue text.