v0.18.0 — locale-aware number and date labels

Minor bump: new public API, no breaking changes. Without a locale every label
is byte-identical to 0.17.1.

Added — locale-aware number and date labels

• ggplot_rs::locale::Locale (re-exported as ggplot_rs::Locale, in the
  prelude and in ggplot_rs::format): decimal mark, thousands mark, minimum
  grouping digits, currency symbol + placement (CurrencyPosition) + spacing,
  percent suffix, short/long month and weekday names, date / month / time
  formats. Presets Locale::en_us(), en_gb(), de_de(), fr_fr();
  Locale::from_tag("de-DE") parses BCP-47 / POSIX tags (de_DE.UTF-8,
  en-GB, …) and falls back to en-US (try_from_tag returns None
  instead); FromStr, Default (en-US). Helpers: localize_number,
  parse_number, format_number, strftime, format_datetime,
  currency_affixes, with_decimal_mark / with_big_mark /
  with_min_grouping_digits / with_currency.
• Formatters with explicit marks: label_number_marks, label_comma_marks,
  label_percent_marks, label_currency_marks; and in a locale's
  conventions: label_number_locale, label_comma_locale,
  label_percent_locale, label_currency_locale (de-DE 1.234,50 €,
  en-US $1,234.50, en-GB £1,234.50, fr-FR 1 234,50 €; de-DE / fr-FR
  percent 12,3 %).
• format::format_value_locale, scale::util::format_numbers_locale /
  format_number_locale (joint break labelling keeps its distinct-labels
  logic, rendered with the locale's marks).
• GGPlot::locale(Locale): plot-wide default for continuous position,
  colour, size and alpha scales (axis ticks and legends, e.g. 1.000 /
  0,5), secondary axes, and date/datetime scales (Okt. 2026 for monthly
  breaks, 15. Okt. 2026 for daily ones, localised %b %B %a %A).
  ScaleContinuous::with_locale / ScaleDateTime::with_locale set it per
  scale (and win over the plot locale); custom label_* formatters win over
  both.
• Scale::set_default_locale / Scale::locale (default methods, no-op /
  None) and ScaleSet::set_default_locale.
• strftime subset gained %a %A (weekday) and %-d %-m (unpadded).