adds EventType consts representing the X-Github-Event header values

[MaerF0x0]

What: for each of the mapped types in github/messages.go it adds a EventType const .

Why: This will help with doing code like

Notes:

• Should be a non-breaking change, exporting many new symbols
• Let me know if you'd rather making it a breaking change on the EventForType function
• Also let me know if you'd like me to define type EventTyper interface{ EventType() EventType } and implement for each of the types

switch eventType {
  case github.EventTypeIssues:
      // stuff
   case github.EventTypeIssueComment, github.EventTypePullRequestReviewComment:
     // comment stuff
}

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