Better features support

[texodus]

This PR refactors <perspective-viewer> to move some engine configuration from compile-time to run-time (via an elaborated Client::get_features call), which in the future may make it easier to implement alternative engines. Perspective enables all features by default and there is no public API to disable them, so this change is mostly UX transparent (except as noted):

• Aggregates function names and filter operators have been moved to Client::get_features.

• Table::validate_expressions has been gated behind the expressions feature, as has the expression UI.

• "filter", "sort", "split_by" and "group_by" UI have been conditioned on Client::get_features named after their config property names.

• Real-time notifications have been conditioned on the on_update feature.

• Minor breaking change - the format of Client::get_features has been changed. However, only new keys have been added ("aggregates", "filter_ops" and "on_update" and "sort")and I suspect this method to be rarely (if ever) used outside of perspective-viewer itself.

• Minor breaking change - the config format for multi-argument aggregates weighted mean, max by and min by has been changed, old:

   const view = await table.view({
      group_by: ["z"],
      aggregates: { x: ["weighted mean", "y"] },
      columns: ["x"],
  });

  New:

   const view = await table.view({
      group_by: ["z"],
      aggregates: { x: ["weighted mean", ["y"]] },
      columns: ["x"],
  });

  This is necessary to allow 3+ parameter aggregates.