[whatwg/xhr] `FormData` `has()` should include a second param for matching the value like `URLSearchParams` (Issue #392)

### What problem are you trying to solve?

`FormData` objects can have multiple entries with the same key. [The `FormData` `has()` method](https://xhr.spec.whatwg.org/#dom-formdata-has) is useful checking a key exists in the object, however, when there are multiple keys it is less useful since in that case you don’t just care about the key, but also if a key and value are present.

### What solutions exist today?

Today, it’s not terribly difficult to achieve this, but it does require getting all of the values as an array and then using `Array` methods to check if the value exists.

For example, where <var>formData</var> is a `FormData` object:

```javascript
formData.getAll("fruit").includes("tomato");
```

### How would you solve it?

[The `URLSearchParams` `has()` method supports a second parameter for matching the value of the named param](https://url.spec.whatwg.org/#dom-urlsearchparams-has). For example, where <var>searchParams</var> is a `URLSearchParams` object:

```javascript
searchParams.has("fruit", "tomato"); // returns a boolean if there’s a match
```

This would be useful for `FormData` as well. So, again where <var>formData</var> is a `FormData` object and the `has()` method supports a second parameter for matching the _name_ and _value_ in the object:

```javascript
formData.has("fruit", "tomato"); // returns a boolean if there’s a match
```

Additionally, this would help authors as there would be increased API parity between `FormData` and `URLSearchParams`.

### Anything else?

_No response_

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