[whatwg/webidl] Support syntax for specifying that an optional record<K, V> has a default value of an empty record<K, V> (Issue #1566)

weinig created an issue (whatwg/webidl#1566)

### What problem are you trying to solve?

There is currently syntax to specify a default value empty `sequence` (`= []`) and a default value default `dictionary` (`= {}`), but no way to specify a default value empty `record<K, V>`.

This is needed to eliminate the invalid syntax used by WebGPU's [GPUDeviceDescriptor](https://gpuweb.github.io/gpuweb/#gpudevicedescriptor), which tries to the default `dictionary` (` = {}`) to default initialize the `record<K, V>` member [requiredLimits](https://gpuweb.github.io/gpuweb/#dom-gpudevicedescriptor-requiredlimits):

```idl
dictionary GPUDeviceDescriptor
         : GPUObjectDescriptorBase {
    sequence<GPUFeatureName> requiredFeatures = [];
    record<DOMString, (GPUSize64 or undefined)> requiredLimits = {};
    GPUQueueDescriptor defaultQueue = {};
};
```

(I have also filed https://github.com/gpuweb/gpuweb/issues/5529 on WebGPU to remove the invalid syntax). 

### What solutions exist today?

_No response_

### How would you solve it?

_No response_

### Anything else?

_No response_

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