Provide `type(C).functionSelectors`

## Abstract

For implementing deployments and upgrades for ERC-2535 Diamonds it is necessary to get all the function selectors of a contract so they can be added to the diamond proxy.

For example here is the struct that needs be filled in order to upgrade or deploy a diamond:
```Solidity
enum FacetCutAction {Add, Replace, Remove}
// Add=0, Replace=1, Remove=2

struct FacetCut {
address facetAddress;
FacetCutAction action;
bytes4[] functionSelectors;
}
```

## Motivation

It would be really useful if it was possible to fill this struct like this:
```Solidity
FacetCut memory facetCut = FacetCut(
address(MyContract),
FacetCutAction.Add,
type(MyContract).functionSelectors)
)
```

Code like that would be used in Foundry to deploy diamonds and make upgrades. Such code could also be useful in factory contracts that deploy diamonds.

This functionality is somewhat similar to `type(I).interfaceId`.

No source files or tests are named in the issue. Start by locating the existing implementation and tests for type(I).interfaceId, then determine the compiler behavior and coverage needed for type(C).functionSelectors; done means the expression provides the selectors required by the FacetCut example.

Written by the indexing model from the issue text.