WIP: update to go mod

[coolaj86]

Fixes #417

• Update to use go.mod as /v3.
• Accidentally ran Prettier on README.md
  • added .prettierrc for standard CommonMark syntax on purpose.

This shouldn't break any existing go.mod with v3.x+incompatible, but re-running go mod tidy in such a repo may or may not have unintended consequences.

The good news is that it's quite simple to fix:

sed -i 's|"github.com/sendgrid/sendgrid-go|"github.com/sendgrid/sendgrid-go/v3' *.* */*.* */*/*.* */*/*/*.* */*/*/*/*.*

It may be worth bumping to /v4, but since the sendgrid API is v3... not sure.

Checklist

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

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[shwetha-manvinkurke]

Closing as a duplicate of #441