chore: mini refactorings and dead code removal

[Nerixyz]

The PR does a few things:

• Removes ConcurrentMap.hpp (2x: util/ and common/)
• Removes util/Clamp.hpp (uses C++ 17's std::clamp)
• Removes util/SplitCommand.{c,h}pp (uses QProcess::splitCommand, 5.15+)
• Removes util/Overloaded.hpp (we only use util/Variant.hpp)
• Removes util/PersistSignalVector.hpp (unused)
• Removes unused code from util/PostToThread.hpp
• Changes chatterino::_helpers_internal to chatterino::helpers::detail
• Adds missing whitespace in DisplayBadge.hpp
• Removes unused <algorithm> include in ExponentialBackoff.hpp
• Changes the initialization from constructor → members for step_ in ExponentialBackoff.hpp (clang-tidy complained that step_ was uninitialized (?))
• Adds IWYU paragma: export to PubSubMessages.hpp (sadly clangd doesn't seem to pick this up in files using the header)
• Converts LayoutCreator to use concepts (should make it a bit easier to read)
  • Replaces T2 with U - U being the newly created/returned type
  • Introduces requirement for addSpacing to be called on a QLayout (widgets don't support that)
  • Simplifies addItem (a bit)