refactor(clock): remove struct waybar_time

The structure was used to pass the locale instance to the date
formatter. All the supported versions of `fmt` are passing the locale
parameter via `FormatContext.locale()` so we can remove the struct and
simplify the code.

While we at it, drop `date::make_zoned` in favor of CTAD on a
`date::zoned_time` constructor.

include/util/date.hpp

@@ -0,0 +1,34 @@
+#pragma once
+
+#include <date/tz.h>
+#include <fmt/format.h>
+
+template <typename Duration, typename TimeZonePtr>
+struct fmt::formatter<date::zoned_time<Duration, TimeZonePtr>> {
+  std::string_view specs;
+
+  template <typename ParseContext>
+  constexpr auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    auto it = ctx.begin();
+    if (it != ctx.end() && *it == ':') {
+      ++it;
+    }
+    auto end = it;
+    while (end != ctx.end() && *end != '}') {
+      ++end;
+    }
+    if (end != it) {
+      specs = {it, std::string_view::size_type(end - it)};
+    }
+    return end;
+  }
+
+  template <typename FormatContext>
+  auto format(const date::zoned_time<Duration, TimeZonePtr>& ztime, FormatContext& ctx) {
+    if (ctx.locale()) {
+      const auto loc = ctx.locale().template get<std::locale>();
+      return fmt::format_to(ctx.out(), "{}", date::format(loc, fmt::to_string(specs), ztime));
+    }
+    return fmt::format_to(ctx.out(), "{}", date::format(fmt::to_string(specs), ztime));
+  }
+};