gh-38592: Add is_integral method to algebraic numbers
For symmetry with existing methods e.g. `QQ(1).is_integral` or such
method on elements of number fields.
Implementation pretty much copied from `NumberFieldElement`.
### :memo: Checklist
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preview.
URL: https://github.com/sagemath/sage/pull/38592
Reported by: user202729
Reviewer(s): Kwankyu Lee, user202729
