enhance: locale matching distance, fallback

[mokurin000]

The default max_distance of langcodes.closest_supported_match 25 is insufficient to match various languages.

=== langcodes ===
max_distance: 25
zh-Hant    -> None
zh-TW      -> None
zh-SG      -> zh-Hans
ja         -> ja
ja-JP      -> ja
ko         -> None
ru         -> None
de         -> None
fr-FR      -> None
it         -> None
en-GB      -> en-US
ms-MY      -> None

max_distance: 100
zh-Hant    -> zh-Hans
zh-TW      -> zh-Hans
zh-SG      -> zh-Hans
ja         -> ja
ja-JP      -> ja
ko         -> None
ru         -> None
de         -> en-US
fr-FR      -> en-US
it         -> en-US
en-GB      -> en-US
ms-MY      -> en-US

max_distance: 200
zh-Hant    -> zh-Hans
zh-TW      -> zh-Hans
zh-SG      -> zh-Hans
ja         -> ja
ja-JP      -> ja
ko         -> en-US
ru         -> en-US
de         -> en-US
fr-FR      -> en-US
it         -> en-US
en-GB      -> en-US
ms-MY      -> en-US

This PR also added a fallback method to make langcodes an optional dependency.

It's now gated behind the i18n-match optional group.

[mokurin000]

@cosven 我重写了一份新的 fallback 逻辑，并且适当调整了 distance 阈值

[cosven]

With max_distance=200, unrelated languages tie against the supported set and langcodes picks the first directory returned by os.listdir. In this PR worktree, supported locales are ["ja-JP", "zh-CN", "en-US"], so t("playlist", locale="ko-KR") and locale="ru" return Japanese instead of falling back to English. The old behavior returned None from closest_supported_match, then loaded the fallback chain. This needs an explicit fallback policy for unrelated languages, or a lower/conditional threshold, before enabling the optional langcodes path.