Add this variable to trigger-based templates

[akloeckner]

Proposed change

This also adds the this variable to trigger-based templates. The code is basically taken from automation and implanted here.

follow-up for #65201

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue: Template variable error: 'this' is undefined when rendering #70359
• Link to documentation pull request: Add this variable to trigger-based templates home-assistant.io#22879

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

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• Documentation added/updated for www.home-assistant.io

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• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
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[probot-home-assistant]

Hey there @PhracturedBlue, @tetienne, @home-assistant/core, mind taking a look at this pull request as it has been labeled with an integration (template) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[akloeckner]

@emontnemery, I believe you were quite active in this field, recently. So, maybe, you're to be involved naturally...

[github-actions]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days.
Thank you for your contributions.

[akloeckner]

.

[emontnemery]

Nice, thanks @akloeckner 👍