refactor: 💡 more type enhancement using built-in keyword

[okinawaa]

I made objectKeys which can infer type strictly by behaving identical like Object.keys() function.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/falso/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

In the original code, when data was added in json file, two file had to be modified, but through type inference, even if data is added, only one place(json file) needs to be modified. This increased the ease of maintenance.

Issue Number: N/A
reponed by #324 because of git issue😂

What is the new behavior?

same as before but increased the ease of maintenance.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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