fix: update @context property append

[julianlam]

The @context property can be a number of types, either a string, or an array containing either strings, or objects.

I don't know how to accurately specify the type without causing warnings in the type-checking logic below, but this is likely due to inexperience, hence why unknown is used.

Original requirement read:

Add the following to object.@context: { "Emoji": "http://joinmastodon.org/ns#Emoji" }

Should read

Append the following to object.@context: { toot: "http://joinmastodon.org/ns#", "Emoji": "toot:Emoji" }

[julianlam]

Per this topic, it's ok to just append without checking if a similarly namespaced object exists, so no additional logic is necessary, I'll remove the comment within.