refactor(eventsub): enforce static error categories

[Nerixyz]

Pop quiz: is there a requirement for the error_category passed to the error_code constructor (docs)?

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Doesn't look like it. boost::system::error_category doesn't say anything either (maybe I missed something). Well, turns out, error_category instances are treated like singletons. So we shouldn't construct them at runtime. One additional constraint we'd like to enforce is that they're supposed to be constant initialized (without the implicit lock inserted by the compiler for statics). We can achieve both of these by constructing a single global instance of the category and by using std/boost::source_location.

Initially, I had some funky workarounds for not being able to pass strings through the code (first commit). However, I realized that the location in the file where an error was emitted from contains the same (if not more) information. This is also simpler, because it's encouraged by boost::system::error_code.

[pajlada]

This type of error seems fine to me 👍

Missing required key (/home/pajlada/git/chatterino/chatterino2/lib/twitch-eventsub-ws/src/generated/payloads/channel-moderate-v2.cpp:228:53 in function 'boost::json::result_for<Vip, boost::json::value>::type chatterino::eventsub::lib::payload::channel_moderate::v2::tag_invoke(boost::json::try_value_to_tag<Vip>, const boost::json::value &)')

[pajlada]

Since we use mostly generated code, we don't have to use the macro if it's causing issues