⚡️ Drop unneeded `BigInt` check in `mixedCase`

[dubzzz]

Description

While not being really expensive to compute it's yet another check and dropping it could be useful if we want to go faster. Indeed with BigInt being a thing in Node since 10.5.0, we probably don't want to pay for checking it all the time.

We will probably have to bump our minimal requirement to 10.5.0+ for the version 4 of fast-check so that we can expect it to always be a thing no matter the runtime being used as long as it fits our requirements.

Checklist — Don't delete this checklist and make sure you do the following before opening the PR

• [x] The name of my PR follows gitmoji specification
• [x] My PR references one of several related issues (if any)
  • [x] New features or breaking changes must come with an associated Issue or Discussion
  • [x] My PR does not add any new dependency without an associated Issue or Discussion
• [x] My PR includes bumps details, please run yarn bump and flag the impacts properly
• [x] My PR adds relevant tests and they would have failed without my PR (when applicable)

Advanced

• [x] Category: ⚡️ Improve performance
• [x] Impacts: No

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👋 A preview of the new documentation is available at: http://66bb0d9b7cf6acd4c376ec5f--dubzzz-fast-check.netlify.app