Add context information to extractors

__Is your feature request related to a problem? Please describe__

Currently, all extractors retrieve the variables from a specific place. E.g. `extractFormFields` considers variables from camunda form fields.

It would be great if, after all variables got collected, we can determine in which context the variable was found. This would make it easier to offer navigation to the place the variable got created.

__Describe the solution you'd like__

I could imagine another field called `context` or `type` or `extractedFrom` when [creating the process variable information](https://github.com/bpmn-io/extract-process-variables/blob/1048507c9d8027243d8d826286764456ca5ba3c2/src/util/ProcessVariablesUtil.js#L37).

```js
{
name: 'myVariable,
origin: [ 'StartEvent_1' ],
scope: 'Process_1',
extractedFrom: 'camunda:formField`
};
```

__Describe alternatives you've considered__

Update data structure to handle origin element and type together, as we have [inside our prototype](https://github.com/pinussilvestrus/variables-prototypes/blob/main/packages/variables-modeler/src/features/variable-store/VariableStore.js#L24-L37).

```js
{
name: 'myVariable,
created: [
{
element: 'StartEvent_1',
type: 'camunda:formField'
}
],
used: [], // skip for now
scope: 'Process_1',
extractedFrom: 'camunda:formField`
};
```

__Additional context__

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