`preallocate = N` setting for `#[derive(Component)]`

## What problem does this solve or what need does it fill?
This fills the role of [`Vec::with_capacity()`](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.with_capacity), but for a component's storage table.

## What solution would you like?
This parameter would cause the backing storage table of a component to start with enough reserved capacity for (at least) N instances of the component.

## What alternative(s) have you considered?
I do not know of any alternative way of doing this - especially as [`bevy_ecs::storage::Table`](https://docs.rs/bevy_ecs/latest/bevy_ecs/storage/struct.Table.html) does not include any way of affecting the capacity of a table.

## Additional context
`bevy_ecs::storage::Table` already exposes the allocated capacity of a table via [`.entity_capacity()`](https://docs.rs/bevy_ecs/latest/bevy_ecs/storage/struct.Table.html#method.entity_capacity). In turn, this can be used in a manner similar to [`Vec.capacity()`](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.capacity) - though I struggle to see how it'd be useful without any meaningful method of affecting the capacity (aside from, of course, attaching the component to more entities).