Investigate/discuss removing 'fieldref' type

[christopherswenson]

From TODO https://github.com/malloydata/malloy/blob/main/packages/malloy/src/lang/ast/field-space/reference-field.ts#L63-L65

The 'fieldref' type seems unnecessary... it obscures the actual type of the field and is redundant with the slightly more verbose { e: [{ type: 'field', path }] }.