refactor: extract utf8 truncation logic to utility

Move the UTF-8 visual-width measurement and truncation helper functions
from the mpris module to a common utility
(include/util/utf8_string.hpp and src/util/utf8_string.cpp).

This decouples string truncation and width measurement
from the mpris module, allowing other modules (like mpd) to reuse.
The helper function `utf8_truncate` (formerly `truncate` in mpris)
and `utf8_width` is exported under the `waybar::util` namespace,
while the low-level `measure_and_truncate` (formerly `utf8_truncate`)
is encapsulated in an anonymous namespace in src/util/utf8_string.cpp
to avoid unnecessary API exposure.

No functional changes were made to the mpris module's behavior.

include/util/utf8_string.hpp

@@ -0,0 +1,7 @@
+#pragma once
+#include <string>
+
+namespace waybar::util {
+size_t utf8_width(const std::string& str);
+void utf8_truncate(std::string& s, const std::string& ellipsis, size_t max_len);
+}  // namespace waybar::util