Prevent modifying Result, GroupResult and FilterResult returned from Filtering

[robertpainsi]

The Result relevant objects are created by Filtering and shouldn't be modifiable.

Currently, not always a copy is returned when using getters, e.g.

filtering.js/src/schema.ts

Line 161 in 340ac26

return this.#groups.get(groupName);

, for performance reason.

It's also okay to discuss other solutions first.