Provide number widget in form

[dafeder]

Re-roll/add test coverage for #4069 by @stefan-korn

QA steps

Add a field to the dataset.json schema file:

    "number": {
      "title": "Number",
      "description": "A unique number for the dataset or API as maintained within an Agency catalog or database.",
      "type": "string",
      "minLength": 1
    }

now add settings to dataset.ui.json:

  "number": {
    "ui:options": {
      "widget": "number",
      "step": 10
    }
  }

Go to create dataset form, confirm there is a number widget that increments by 10 on every click.

[kaise-lafrai]

Looks good and QA steps passed. One small request is to update the code comment for the numberField in the WidgetRouterTest.