Migrate to C++ 20 & switch to websocketpp develop branch (#4252)

* feat: c++ 20

* fix: c++ 20 deprecations

* fix(msvc): warnings

* chore: add changelog entry

* fix: formatting

* Update websocketpp to the `develop` branch

* Specify other template type in FlagsEnum != operator

* Remove the user of simple template ids in our websocketpp template class

Also standardizes the file a bit by using nested namespaces, using
pragma once

* fix: turn `MAGIC_MESSAGE_SUFFIX` into a `QString`

* hacky unhacky hacky const char hack

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/common/CompletionModel.cpp

@@ -94,7 +94,7 @@ void CompletionModel::refresh(const QString &prefix, bool isFirstWord)
     // Twitch channel
     auto *tc = dynamic_cast<TwitchChannel *>(&this->channel_);
 
-    auto addString = [=](const QString &str, TaggedString::Type type) {
+    auto addString = [=, this](const QString &str, TaggedString::Type type) {
         // Special case for handling default Twitch commands
         if (type == TaggedString::TwitchCommand)
         {

src/common/FlagsEnum.hpp

@@ -32,7 +32,7 @@ public:
         return this->value_ == other.value_;
     }
 
-    bool operator!=(const FlagsEnum &other)
+    bool operator!=(const FlagsEnum<T> &other)
     {
         return this->value_ != other.value_;
     }