feat(yaml/unstable): export `unstable_stringify.ts`

[uki00a]

yaml/unstable_stringify.ts was added in #6666. This PR exports it to make it available to users.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.68%. Comparing base (3d44532) to head (55fd883). Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6702      +/-   ##
==========================================
- Coverage   94.70%   94.68%   -0.02%     
==========================================
  Files         567      567              
  Lines       46793    46793              
  Branches     6582     6582              
==========================================
- Hits        44315    44308       -7     
- Misses       2436     2442       +6     
- Partials       42       43       +1     

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