modernization: tweaks for c++20

Update various places for what we can do with C++20:

- ends_with / starts_with
- std::to_array
- using instead of typedef
- designated initializers
- "[[maybe_unused]]" instead of G_GNUC_UNUSED

Also remove some unnecessary 'static'

lib/mu-query-parser.cc

@@ -54,9 +54,9 @@ static bool
 looks_like_matcher(const Sexp& sexp)
 {
 	// all the "terminal values" (from the Mu parser's pov)
-	const std::array<Sexp::Symbol, 5> value_syms = {
+	const auto value_syms = std::to_array({
 		placeholder_sym, phrase_sym, regex_sym, range_sym, wildcard_sym
-	};
+	});
 
 	if (!sexp.listp() || sexp.empty() || !sexp.front().symbolp())
 		return false;