[Polaris] - Theme switcher on local and testing envs

[Powerplex]

• Implemented a PolarisThemeSelector on Polaris, only visible in local and test envs, to easility switch between brands themes.
• Implement a PolarisThemeProvider and its useTheme hook, which allows developers to switch the global theme of the app by calling setTheme(myTheme)

Test link: https://spark-theme-switched.next.ariane.leboncoin.ci/

=> Check the button to the bottom right of the screen.

[thomas-leguellec]

Topic took by a global taskforce which will manage all the i18n and not only DesignSystem