[RFE] user defined global sub templates

[nocturnalastro]

Is your feature request related to a problem? Please describe.
I find there are common bits of logic that I wish to have shared across templates.

Describe the solution you'd like
The ability to register a set sub-template definitions

{{ define "foo" }}
...
{{ end }}

These are added to each tree of the parsed so they can be used in all templates.

Describe alternatives you've considered
None

Additional context
I work on another project which has a similar feature which we implement here and I'm happy to help implement this if you want.

[github-actions]

This issue has become stale and will be closed automatically within 7 days.

[nocturnalastro]

@rsevilla87 @vishnuchalla
What do you think?