WIP: Initial support for translation

[sylvestre]

WIP: Initial support for translation. It uses fluent and can be enabled with the feature "i18n" And incldues the French translation as poc.

If not built with this feature, it should not add any overhead as it will take the english string directly in the code.

TODO:

• fix a better path for the locale files
• ship the resources in the binary
• fix some tests failing
• find the right versions of the new dependencies to preserve the MSRV
• Add tests to verify that it is indeed translated
• Update of the doc

closes #380

[sylvestre]

@epage @alerque before I spend more time on it, is that an approach that you agree with ? thanks

[epage]

The conversation for the general direction belongs on #380 and I posted at https://github.com/clap-rs/clap/issues/380#issuecomment-2976940316