[12.x] feat: query builder aliases for expressions

[tpetry]

Currently, you can add subqueries to a query builder with an alias.

// select (select "allowed_to_x" from "configuration") as "sub" from "users"
User::select([
    'sub' => Configuration::select('allowed_to_x'),
]);

But the same is not yet possible for expressions There's just no way to define an alias. This PR fixes this

// current: select available_a - used_a from "users"
// with pr: select (available_a - used_a) as "remaining_a" from "users"
User::select([
    'remaining_a' => new Expression('available_a - used_a'),
]);

---

So this PR changes:

• adds selectExpression (like selectSub)
• adds expression alias support to select
• adds expression alias support to addSelect
• adds units to test the behaviour of array keys for select and addSelect which was not covered yet

[tpetry]

The failing static analysis check is because of some blade compiler stuff. Not touched that part.