Allow a default Schema.org tree per bundle (content type, vocabulary…)

Problem/Motivation

Currently the Free Schema.org tree can only be defined per entity: editors have to build (or import) the tree on every node or taxonomy term. On sites with many entities of the same bundle this is repetitive and error-prone, even though the structure is usually the same for all of them (e.g. every `event` node is an `Event` with the same properties, and every term of the `topics` vocabulary is a `DefinedTerm` or `CollectionPage`), and only the values change.

Since tree values already accept tokens (`[node:title]`, `[term:name]`, `[node:url:absolute]`...), a single tree defined once per bundle would be enough to produce correct JSON-LD for all its entities.

Proposed resolution

Add a bundle-level **default tree**, configurable for any fieldable entity bundle (node types, vocabularies and other content entity bundles):

• Add a "Free Schema.org" section to the bundle edit form (e.g. `/admin/structure/types/manage/{type}`, `/admin/structure/taxonomy/manage/{vocabulary}/overview`), using the same tree editor widget (type/property suggestions, JSON-LD import, token browser).
• Store the default tree as a third-party setting of the bundle config entity (`free_schema` key), so it is exportable with configuration management and translatable via config translation.
• At render time, the JSON-LD for an entity is resolved as follows:
  1. If the entity has its own value in a Free Schema.org field, use it (current behaviour).
  2. Otherwise, if its bundle has a default tree, use it, replacing tokens with the current entity.
  3. Otherwise, do nothing (other JSON-LD on the page is left untouched).
• The default tree replaces every other JSON-LD on the page, exactly like a per-entity tree.
• Editing the default tree requires the existing restricted permission (or a new "Administer free Schema.org defaults" permission).
• Optionally, show the inherited default as a read-only preview / starting point in the entity widget, so editors can see what will be printed and copy it to override it.