chore!: rename top-level lib folder to `open_feature`

[maxveldink]

This PR

BREAKING CHANGE: Since we've decided on OpenFeature as our top-level module name, we should follow the Ruby convention of separating the words with an underscore in the file name.

Related Issues

References #71

Follow-up Tasks

• Think about renaming the package name, more discussion in #71

[codecov[bot]]

Codecov Report

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

Comparison is base (60423be) 98.90% compared to head (c40715f) 98.90%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #90 +/- ## ======================================= Coverage 98.90% 98.90% ======================================= Files 6 6 Lines 91 91 ======================================= Hits 90 90 Misses 1 1 ```

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