Add HasSeatbeltOn export to seats.lua

[BackSH00TER]

Describe Pull request
Adds a simple HasSeatBeltOn export to the seatbelt.lua file.
This returns the state of the seatbeltOn variable that is maintained from this file.

This enables other scripts to use this for whatever they need.

Questions (please complete the following information):

• Have you personally loaded this code into an updated qbcore project and checked all it's functionality? [yes]
• Does your code fit the style guidelines? [yes]
• Does your PR fit the contribution guidelines? [yes]