Validator do not allow customError

@christophe-g is already working on this.

Since Dec 15, 2023.

### What is affected?

Accessibility, Component

### Description

A validator using `setCustomValidity` as below will never cause an invalid event to be dispatched.

```ts
export class MyValidator extends Validator{
private checkboxControl?: HTMLInputElement;

protected override computeValidity(state) {
if (!this.checkboxControl) {
this.checkboxControl = document.createElement('input');
this.checkboxControl.type = 'checkbox';
}
this.checkboxControl.setCustomValidity('Get rid of me');

console.warn('I should be invalid', this.checkboxControl.validity)
return {
validity: this.checkboxControl.validity,
validationMessage: this.checkboxControl.validationMessage,
};
}
```

This is due to the [Safari workaround here](https://github.com/material-components/material-web/blob/926edfb367de71eed77deaba45a4a7eaa79342fc/labs/behaviors/constraint-validation.ts#L243), causing the `customError` flag to always be false.

Otherwise new validators are great

### Reproduction

https://lit.dev/playground/#gist=2fecd21c77070203e5472131ffc9b199

### Workaround

Force an error on another flag, but this is not really nice:

```
protected override computeValidity(state: RecordState) {
if (!this.selectControl) {
// Lazily create the platform select
this.selectControl = document.createElement('input');
}
let customError = '';
if(state.currentDuration > state.maxDuration) {
customError = 'Recording is too long';
}
if(customError) {
// we force an error because validator [syncValidation] has a bug
// with customError
this.selectControl.value = '';
this.selectControl.required = true;
this.selectControl.setCustomValidity(customError);
} else {
this.selectControl.value = state.value;
this.selectControl.required = state.required;
}
return {
validity: this.selectControl.validity,
validationMessage: customError || this.selectControl.validationMessage,
// validationMessage: this.selectControl.validationMessage,
};
}
```

### Is this a regression?

No or unsure. This never worked, or I haven't tried before.

### Affected versions

Failing in v1.1.1

### Browser/OS/Node environment

Latest chrome on linux