Add new flatten method so we can generate notes in semconv

[jsuereth]

This is actually an interesting issue where minijinja does NOT support mutating existing lists via append, but jinja does.

For now, we just use [notes, [newitem]] | flatten to simulate append as a workaround.

[codecov]

Codecov Report

Attention: Patch coverage is 14.28571% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 63.2%. Comparing base (ec0d0c7) to head (1cecd05).

❗ Current head 1cecd05 differs from pull request most recent head 48fdc6c. Consider uploading reports for the commit 48fdc6c to get more accurate results

Files	Patch %	Lines
crates/weaver_forge/src/lib.rs	14.2%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #80     +/-   ##
=======================================
- Coverage   63.3%   63.2%   -0.2%     
=======================================
  Files         54      54             
  Lines       3202    3209      +7     
=======================================
+ Hits        2030    2031      +1     
- Misses      1172    1178      +6     

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