Give ids to array and object fields

[epicfaace]

This change:

• Gives ids to the <fieldset> tag in the default array field template and object field template
• Gives the id of "root" to the topmost <fieldset> in the form.

Reasons for making this change

This way, we are more consistent with the idSchema, so that every id mentioned in the idSchema actually has a corresponding element rendered in the DOM. This allows us to create anchor links that go to specific elements, even if they are not <input> or <select> elements.

For example, on the "Arrays" example in the playground, here is the idSchema:

However, there are no elements with ids equal to "root", "root_fixedItemsList", etc. This change fixes that.

This change was especially motivated by looking into #86 -- sometimes you may have a validation error that refers to an array element (such as minimum items not being reached). If there is no corresponding DOM ID corresponding to the idSchema value for that element, then we cannot create anchor links that go to the specific element with the error.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
  • If I'm adding a new section, I've updated the Table of Content
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
  • I've run npm run cs-format on my branch to conform my code to prettier coding style
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[glasserc]

Thanks for the great PR description!

[glasserc]

Also, sorry for taking so long to get to this!