Implement is_square for LaurentSeries

[Hapsa21]

Description

This PR implements the missing is_square() method for LaurentSeries and LazyLaurentSeries elements. Previously, calling .is_square() on these series would raise an AttributeError.

The implementation follows the mathematical definition: a Laurent series $f = t^v \cdot u$ is a square if and only if:

1. The valuation $v$ is even.
2. The unit part $u$ is a square (in the underlying PowerSeriesRing or base ring).

Changes:

• LaurentSeries: Implemented is_square by checking the valuation and delegating the unit check to the underlying PowerSeries logic.
• LazyLaurentSeries: Implemented is_square to harmonize functionality, ensuring it handles unit shifts correctly before delegating to the lazy sqrt() method.

Both implementations support the root=True argument to return the square root when it exists.

Fixes

#41221

[mantepse]

It would be great to harmonize with lazy Laurent series.

[Hapsa21]

It would be great to harmonize with lazy Laurent series.

done

[tscrim]

We generally do not have periods/full-stops at the end of input blocks. Once changed, I will set a positive review.

[tscrim]

Okay, LGTM.

(Mostly for @mantepse) At some point, we probably should figure out a way to combine stuff for both classes; perhaps some kind of mixin class.