chore: flatten modules to make simpler; update dependabot file

[markphelps]

• Flatten some modules to the root to make jumping around a bit easier
• Update dependabot to include flipt-client-browser and flipt-engine-wasm correctly
• Move an FFI only error into the flipt-engine-ffi lib

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.38%. Comparing base (4bf3db8) to head (590094f).

Files	Patch %	Lines
flipt-engine-ffi/src/lib.rs	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #316 +/- ## ========================================== - Coverage 88.41% 88.38% -0.03% ========================================== Files 10 10 Lines 3685 3686 +1 ========================================== Hits 3258 3258 - Misses 427 428 +1 ```

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