refactor(dotenv): avoid recursive `expand()`

[timreichen]

This PR refactors the expand() fn so it uses a loop instead of calling it recursively. This makes it much more readable.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 94.10%. Comparing base (2f99801) to head (1d17d2a). :warning: Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6892      +/-   ##
==========================================
- Coverage   94.11%   94.10%   -0.01%     
==========================================
  Files         582      582              
  Lines       42752    42747       -5     
  Branches     6808     6806       -2     
==========================================
- Hits        40236    40229       -7     
- Misses       2466     2467       +1     
- Partials       50       51       +1     

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[kt3k]

BTW it looks like deno run --env-file does not support default value substitution in variable expansion