New variable `{{yaml_content}}`

[Taitava]

Discussed in https://github.com/Taitava/obsidian-shellcommands/discussions/57

^{Originally posted by Taitava September 30, 2021}

• {{yaml_content}}: Give only YAML.

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TODO:

• [ ] Create {{yaml_content}} variable
• [ ] Create {{event_yaml_content}} variable
• [ ] Write documentation
• [ ] Make tests

I haven't decided a milestone yet, as Variable.isAvailable() nees to be refactored to use Promises (make a new issue for this, perhaps?).