Allow editing template part names and categories from the editor

[Sarah Norris]

We should consider allowing users to edit template part names and categories and then save these changes to the current theme.json file.

e.g. This part of theme.json:

	"templateParts": [
		{
			"area": "uncategorized",
			"name": "comments",
			"title": "Comments"
		},
		{
			"area": "header",
			"name": "header",
			"title": "Header"
		},
		{
			"area": "footer",
			"name": "footer",
			"title": "Footer"
		}
	],

[Grant Kinney]

It looks like you can name the template part and set the area when it's created:

But when exporting the changes to the theme with Create Block Theme, only the template part file is created. The name, title, and area of the template part are not added to the templateParts key of theme.json.