Add keyPrefix support to getFixedT in RemixI18Next

[sergiodxa]

Related to #142

Adds support for the keyPrefix option in the getFixedT function within RemixI18Next, aligning server-side translation capabilities with the original i18next and client-side usage.

• Updates the method signature of getFixedT in src/server.ts to accept an additional keyPrefix parameter.
• Implements logic within getFixedT to prepend the provided keyPrefix to translation keys, enhancing namespace management and translation key organization.
• Modifies the README.md to document the new keyPrefix option, including usage examples and its benefits for server-side translation scenarios.

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