refactor: common/Credentials (#4979)

Use full path in includes
Sort includes
Move anon namespace out of chatterino namespace
Use auto * where possible
Disable convert-member-function-to-static check for all member functions
Don't use else after return
Removed empty constructor
Replace use of `boost::variant` with `std::variant`

Co-authored-by: nerix <nerixdev@outlook.de>

src/common/Credentials.hpp

@@ -19,7 +19,7 @@ public:
     void erase(const QString &provider, const QString &name);
 
 private:
-    Credentials();
+    Credentials() = default;
 };
 
 }  // namespace chatterino