fluent-syntax: Allow Placeables inside other Placeables

[stasm]

This should be possible:

key1 = {{ foo }}
key2 = {     { foo }     }
key3 = 
    {
        { foo }
    }

[zbraniecki]

What's the purpose?

[stasm]

Grouping: projectfluent/fluent#3

[zbraniecki]

Is that an artifact of the EBNF grammar or do you expect that anyone will actually have a reason to try to write FTL using this?

[stasm]

projectfluent/fluent#3 has the use-case: list-typed selectors.

[stasm]

This was fixed in #287.