* feat: make this variable available in template entities
This makes the variable `this` available in template entities.
It will simplify the use of self-referencing template entities.
Because, without this, we have to repeat the entity id every time.
If we can solve this without explicitly spelling the entity id,
code can be re-used much better.
As a side-effect, this will allow to use `variables`-like patterns,
where attributes can be used as variables to calculate subsequent attributes or state.
Example:
```yaml
template:
sensor:
- name: test
state: "{{ this.attributes.test }}"
# not: "{{ state_attr('sensor.test', 'test' }}"
attributes:
test: "{{ now() }}"
```
* expose entity_id instead of this
* add test
* Refactor to expose this variable
* Tweak repr dunder
Co-authored-by: Erik <erik@montnemery.com>
d20a620590