[WIP] fexpr unary operators

[samukweku]

• WIP Refactor Expr architecture #2562
• focuses on unary operators

[samukweku]

@oleksiyskononenko i'm stuck on how to adapt the code for unary minus and sorting, i.e dt.sort(-f.A) or dt.by(-f.A)

[oleksiyskononenko]

@samukweku You need to rebase this PR on top of main, otherwise it is showing too many old changes.

[samukweku]

@oleksiyskononenko this is red because I do not know how to adapt the code for unary minus and sorting, i.e dt.sort(-f.A) or dt.by(-f.A). I would appreciate your guidance on this one