Add `LinearModel` and `CompactLinearModel` class objects

[pr0m1th3as]

LinearModel and CompactLinearModel class objects class with conditional support for tables and categorical arrays from the datatypes package should be implemented. Support for tables and categorical data should be extended to the fitlm function, which must be adapted to the return a LinearModel object instead of the STATS structure in its current implementation. A stewiselm function should also be implemented. Tagging Andrew Penn (@acp29) for notification on this one, since he is the original author of the fitlm function.

This is a rather long term goal and as a relatively large project to implement it could be a candidate GSoC project for 2026.