[feature] add previous_price to restock values

[opicron]

Version and OS
v0.54.4 Docker

Is your feature request related to a problem? Please describe.
In my restock price notification I add the previous price as additional information. To make this work I have to add this jinta code to get the previous price from the difference in text:

{% set lines = diff_patch.splitlines() %}
{% if diff and 'Example' not in diff %}{% for line in lines %}{% if line.startswith('-') and 'Price:' in line %}{% if line.split('Price: ')|length > 1 %}Previous amount: {{ line.split('Price: ')[1].split()[0] }}{% else %} {% endif %}{% endif %}{% endfor %}{% endif %}

Describe the solution you'd like
Currently in watch.json the restock prices and availability is stored like so:

  "restock": {
    "in_stock": false,
    "price": 44.99,
    "currency": "EUR",
    "original_price": 44.99,
    "availability": "\n \n \n \n \n \nhttps://schema.org/outofstock\n \n  \n \n \n \n"
  },

Could we add a previous_price value which is saved when a new price is detected? The previous_price value would be the price value just before it is set with the new price.

Describe the use-case and give concrete real-world examples
This would significantly improve the jinta code and provide more information to notifications.

[dgtlmoon]

this makes good sense

[dgtlmoon]

original_price is actually just that, original_price..

infact, it's written clearly in the token list that it's the first price

so hmm

[dgtlmoon]

And here we go - #3993

[opicron]

Very nice approach! Going to check it this weekend now.

[dgtlmoon]

Excellent, I have a few things going on outside of computer-world atm, so i might be slow, but do check it out.

[opicron]

The previous price is showing well. When there is no price it correctly shows None. Thank you very much again.

Happy to see my jinja2 template code reduced to Previous amount: {{restock.previous_price}} instead of the long code I used before.

[dgtlmoon]

Fantastic!