feat: Support external links in navigation

[marek-mihok]

This change adds the path attribute for ui.nav_item.

Updated API:

/** Create a navigation item. */
export interface NavItem {
  /** The name of this item. Prefix the name with a '#' to trigger hash-change navigation. */
  name: Id
  /** The label to display. */
  label: S
  /** An optional icon to display next to the label. */
  icon?: S
  /** True if this item should be disabled. */
  disabled?: B
  /** An optional tooltip message displayed when a user hovers over this item. */
  tooltip?: S,
  /** The path or URL to link to. If specified, the `name` is ignored. The URL is opened in a new browser window or tab. E.g. `/foo.html` or `http://example.com/foo.html` */
  path?: S
}

Screen.Recording.2023-05-02.at.11.09.04.mov

Closes #1952

[mturoci]

Thanks @marek-mihok, just one small nitpick comment.