Implement `Stack.alignment`

[ndonkoHenri]

Discussed in https://github.com/flet-dev/flet/discussions/2799

^{Originally posted by ingdesarr1 March 4, 2024} Good afternoon. It would be very useful if the alignment properties could be used within the Stack, to facilitate aligning the elements within it. For example, if I have an image and a column with texts inside, I could align the column to the center, or to the side. Thank you