chore: add TypeScript Strict Mode Support for the `@asgardeo/javascript` package

[pavinduLakshan]

Subtask of #142

[areebniyas]

Hi @pavinduLakshan, I understand the tight schedule and thank you for getting back. Lets review [1] and close this child issue ASAP. Cheers.

[1] #197

[sagara-gunathunga]

@areebniyas Congratulations on completing this issue! Unless you’ve already done so, please complete the form for this issue as mentioned on the WSO2 Hacktoberfest webpage: https://wso2.com/hacktoberfest/