Feature request: Indexes on Expressions

[ryanheise]

https://www.sqlite.org/expridx.html

Proposed syntax:

@Entity(indices: [
  Index(value: ['y + z']),
])
class Foo {
  @primaryKey
  int x;
  int y;
  int z;

  Foo({
    required this.x,
    required this.y,
    required this.z,
  });
}