feat: support `FLAGD_DEBUG` / `--debug` / `-x`

[snosratiershad]

This PR

• Observes debug env variable [with or prefix will be FLAGD_DEBUG] and binds --debug and -x flags to it.

Related Issues

Fixes #1323

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[codecov[bot]]

Codecov Report

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

Project coverage is 78.74%. Comparing base (1c530ab) to head (b7526bd). Report is 68 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1326 +/- ## ========================================== + Coverage 73.69% 78.74% +5.05% ========================================== Files 32 36 +4 Lines 3140 2828 -312 ========================================== - Hits 2314 2227 -87 + Misses 717 466 -251 - Partials 109 135 +26 ```

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[toddbaert]

Thanks @snosratiershad