Add pure JSON serialization cheatcodes

### Component

Forge

### Describe the feature you would like

Hi everyone!

The current approach to handling JSON feels somewhat suboptimal. Working with the id -> json map feels like an extra step, which, in my opinion, isn't entirely necessary. If the JSON serialization methods were made "pure", they could operate directly on a JSON string, which has the advantages of not modifying state and being more intuitive.

Consider the current method:

```solidity
string memory id = "myObj";
vm.serializeString(id, ".foo", "bar");
vm.serializeString(id, ".hello", "world");
string memory json = vm.serializeUint(id, ".someUint", 123);
```

With a direct JSON string approach, it could be simplified to:

```solidity
string memory json = '{ "foo": "bar" }';
json = vm.serializeString(json, ".hello", "world");
json = vm.serializeUint(json, ".someUint", 123);
```

This method offers more clarity in the way we work with JSON. Also, by the convention used by `forge-std`, in this case the functions could be declared as `pure`. Of course this would break a lot of things... an alternative would be to add these as new methods (not ideal as the vm interface is already pretty big).

Thoughts?

### Additional context

_No response_