fix: add :enabled sub-config for fixer tools This allows distributing opt-in fixer tool configurations in .hgrc files. This may be useful for adding default configs in core, or for orgranizations that want to provide configs to their users. Tools are still enabled by default because it would be confusing to add a config and find that it has no effect until you add enabled=true. Differential Revision: https://phab.mercurial-scm.org/D6975