`.selector` on functions and error definitions in assembly

## Motivation

`.selector` exists on both functions and error definitions. It even exists in one case for functions in assembly: assigning to function types (example [here](https://docs.soliditylang.org/en/v0.8.35-pre.1/assembly.html#access-to-external-variables-functions-and-libraries)). Tried everything on 0.8.34.

It would be nice to have reading the selector for both functions and errors available to assembly. Some libraries (like solady) make use of this for quick error paths (example [here](https://github.com/Vectorized/solady/blob/main/src/accounts/ERC7821.sol#L162)):
```
mstore(0x00, 0x3995943b) // `BatchOfBatchesDecodingError()`.
revert(0x1c, 0x04)
```

## Specification

Support `.selector` in assembly, i.e. the above would be:
```
mstore(0x00, BatchOfBatchesDecodingError.selector)
revert(0x1c, 0x04)
```