feat!: add Go modules and Go 1.19 compatibility

[zwartho]

Fixes

This PR is based off the changes introduced in #112 by @AlaricWhitney.

Adds Go Modules, and also adds a v3 directory as per current Golang standards as per https://go.dev/blog/v2-go-modules.

This PR also makes the following changes:

• Minor adjustments to correct compatibility up to Go 1.20
• Adds new test-v3 job due to breaking changes
• Updates to documentation

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

If you have questions, please file a support ticket.

[zwartho]

@marcoshuck this is ready for review.