fix(init): use JS template for dynamic configuration

[sanskarajput]

Pull Request check-list

Please make sure to review and check all of these items:

• Does npm run test pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Have you added new tests to prevent regressions?
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Description of change

Closes #1566

When using .sequelizerc with a .js config path, sequelize init
was generating JSON content inside the .js file instead of valid JS.

This fix checks if the config path ends with .js and generates
the correct JS template accordingly.