Make formattime follow system locale

[CrumblyLiquid]

Description

Fixes #869.

This draft:

1. Creates new function get_locale in eww_shared_util that returns the chrono Locale enum.
2. Enables the chrono unstable-locales feature for the simplexpr crate
3. Changes the t.format function in simplexpr formattime expression to t.format_localized which then uses the aforementioned get_locale function to get the appropriate Locale enum.

Related issues:

• Strftime %x not localized chronotope/chrono#708
• Time module does not respect locale greshake/i3status-rust#73
• Localized date/time chronotope/chrono#199
• Locales chronotope/chrono#453

Questions

1. Is eww_shared_util a good place to put get_locale into?
2. Is it desirable to have the get_locale function called every time the formattime expression is evaluated?
3. We might also want to check for the LANG variable if LC_TIME fails.
4. Maybe we want to use some different way of getting the format like libc-strftime?

Checklist

Please make sure you can check all the boxes that apply to this PR.

• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing