Add Unicode BiDi issolation characters

[alerque]

For variable substitutions and maybe some other places, we should probably have an option to wrap them as in the Python implementation:

You will notice the extra characters \u2068 and \u2069 in the output. These are Unicode bidi isolation characters that help to ensure that the interpolated strings are handled correctly in the situation where the text direction of the substitution might not match the text direction of the localized text. These characters can be disabled if you are sure that is not possible for your app by passing use_isolating=False to the FluentBundle constructor.