fix: widen flag return types

[andreas-karlsson]

Hi There, I just made a Pull Request!

Right now we have an issue that flag value types are too aggressively "narrowed" by TS. For instance:

const value = await confidence.getFlag('ff', 3);

Right now TS will infer the type of value to be 3 withe the desired type being number. This PR fixes that. Ideally we should have type tests for this sort of thing. But that is saved for another day.

:heavy_check_mark: Checklist

• [ ] All tests are passing
• [ ] Relevant documentation updated
• [ ] linter/style run on changed files
• [ ] Tests added for new functionality
• [ ] Regression tests added for bug fixes
• [ ] Tested in a corresponding example app