Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

Libraries/LibGUI/JSSyntaxHighlighter.cpp

@@ -63,9 +63,8 @@ bool JSSyntaxHighlighter::is_identifier(void* token) const
     return js_token == JS::TokenType::Identifier;
 }
 
-bool JSSyntaxHighlighter::is_navigatable(void* token) const
+bool JSSyntaxHighlighter::is_navigatable([[maybe_unused]] void* token) const
 {
-    (void)token;
     return false;
 }