Add an in-memory-preloading loader for Jinja imports

[depoll]

Breaking change

Proposed change

#88624 was locked, so opening a new PR with a draft of a new approach based on @balloob's feedback. Happy to revise based on feedback. Entirely possible I've chosen the wrong point for initialization, etc.

Breaking out part of https://github.com/home-assistant/core/pull/88575

Enables importing/including shared jinja code/macros from a custom_jinja folder in your config folder.

All of this enables the following code:

In config/custom_jinja/shared.jinja:

{% macro some_macro() -%}
A macro
{%- endmacro %}
{%- set some_constant = "A constant" %}

Elsewhere:

{%- import "shared.jinja" as shared %}
{{ shared.some_constant }}
{{ shared.some_macro() }}

With the following output:

A constant A macro I'm happy to update the documentation as well but wanted to first clear that this would be a welcome change.

Type of change

• [ ] Dependency upgrade
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New integration (thank you!)
• [x] New feature (which adds functionality to an existing integration)
• [ ] Deprecation (breaking change to happen in the future)
• [ ] 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:
• Link to documentation pull request:

Checklist

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

If user exposed functionality or configuration variables are added/changed:

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

If the code communicates with devices, web services, or third-party tools:

• [ ] 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.
• [ ] Untested files have been added to .coveragerc.

To help with the load of incoming pull requests:

• [ ] I have reviewed two other open pull requests in this repository.