i18n fixes + sync

[Klemen2]

The current i18n implementation is quite limited, since I'm making a multi-window project I upgraded it with sync version of a signal

• fixed an issue where if id is not found in the language that exists, it returns empty string instead of fallback
• fixed an issue where if id is not found in the fallback language that exists, it returns empty string instead of id
• added docs
• added sync version for usage with mutex / rwlock for multiwindow applications
• added better fallback support (similar to https://www.i18next.com/principles/fallback)
• added a way to iterate through the languages for easier language selection
• added extra optional field for language name - usually you want to be in a language that the translation is in, not in the displayed language otherwise someone would not be able to read what language they are selecting since they cannot read or understand the language that is currently selected
• added extra optional field for language image - just so it's easier to make add it with iterator

Works in 0.5 for web and windows, cannot test mac or linux

[marc2332]

I suggest you take a look at https://crates.io/crates/dioxus-i18n first and see if that fits you better, we will probably remove i18n support from the sdk as the new crate has more features, let me know what you think