This PR makes a bunch of (mostly cosmetic) changes to bring this repo into this decade:

• It adds a go.mod file, allowing this module to be properly versioned
• It makes a couple of linting updates:
  • prefer double quotes over backticks where backticks aren't necessary
  • prefer myvar := "some value" over var myvar = "some value"
  • use lowercase strings in errors
• Adds a buildkite pipeline