Make `Trigger::entity()` panic on `Entity::PLACEHOLDER` and add `Trigger::get_entity()`

## What problem does this solve or what need does it fill?

Trying to perform commands on trigger entity can lead to errors later than expected (command execution instead of call site).

## What solution would you like?

While `Entity::PLACEHOLDER` works great for the internals, the users should be presented with the standard two methods:
- `entity()` - get a valid entity or panic
- `get_entity()` - get a valid entity or none

This requires a breaking change on `entity()` which currently never panics, but can return `Entity::PLACEHOLDER`.

## What alternative(s) have you considered?

Check for placeholder by oneself.