namespaces: record and expose whether namespace is built-in Currently, the templating layer tends to treat each namespace as a one-off, with explicit usage of {bookmarks}, {tags}, {branch}, etc instead of using {namespaces}. It would be really useful if we could iterate over namespaces and operate on them generically. However, some consumers may wish to differentiate namespaces by whether they are built-in to core Mercurial or provided by extensions. Expected use cases include ignoring non-built-in namespaces or emitting a generic label for non-built-in namespaces. This commit introduces an attribute on namespace instances that says whether the namespace is "built-in" and then exposes this to the templating layer. As part of this, we implement a reusable extension for defining custom names on each changeset for testing. A second consumer will be introduced in a subsequent commit.