docs: update README examples to use const and let

[Planeshifter]

Description

What is the purpose of this pull request?

This pull request:

• updates all example code in README.md files across the project to use let and const declarations instead of the older ES5 var declarations. Given most downstream users of the library will only be familiar with modern JavaScript syntax, this will increase the perception of the project as up-to-date and following best practices.

Related Issues

Does this pull request have any related issues?

No.

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

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