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

[MaerF0x0]

What: Adds type EventType string and an EventType const for every of the types in the maps in github/messages.go

Why: So that we can do switches like

switch eventType {
    case github.EventTypeFork:
        // do stuff
     case github.EventTypeDiscussionComment, github.EventTypeIssueComment:
        // do something with either
}

Notes:

• It should be non-breaking change
• Let me know if you want the EventType used more widely including implementing type EventTyper interface{} on each of those structs

[gmlewis]

Thank you, @MaerF0x0!
Two minor tweaks, please, then we should be ready for a second LGTM+Approval from any other contributor to this repo before merging.

[gmlewis]

Suggested change

	// EventType corresponds to the X-GitHub-Event: header for Webhooks events and payloads
	// EventType corresponds to the X-GitHub-Event: header for Webhooks events and payloads.

[gmlewis]

Since this is not used anywhere, let's please delete it.
If someone finds this useful in their own code, then it is a simple matter to create it there instead of here.

[gmlewis]

Also, please make sure all tests run locally by following step 4 of CONTRIBUTING.md and push any necessary changes to this PR.