Add Competing to activity types

[kaine119]

Summary

Add support for Competing activity type to Activity and Bot. Fixes #14.

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Added

• Activity::COMPETING, ActivitySet#competing
• Bot#competing= to set the competing status
• Updated comments to add Competing to the list of activity types

[kaine119]

@swarley mentioned adding an identification method Activity#competing?, but those are currently used only for activity flags, not for activity types. Should we add identifications for all of those too, or leave it as is?

[swarley]

You were correct to leave it out 😅. I had assumed, for some reason, we had an abstracted way to identify the type.

We can put a pin in that for now, since adding something like that should be its own PR.

[swarley]

Thank you!