Add a config variable with engine support

[sophiajt]

This adds a new builtin variable $config. The $config variable allows users to update the config by setting this to a record with known fields.

I currently have only ported over a couple settings, but we can keep adding more.

My current config.nu:

let $config = {
  filesize_metric: $true
  table_mode: with_love
}

The $config variable follows all the scoping of normal variables and is assumed captured by blocks.

[sophiajt]

Going ahead and landing. We can improve this in future updates, but probably good to porting some more code over to using it.