Example:
```
" {NAME} est {GENDER, select, female {allée } other {allé}} à {CITY}."
```

We trim when tokenizing the contents of a message format:
{allée } => [ 'allée' ]

We don't trim when tokenizing the pattern string:
[
    ' ',
    '${NAME}',
    ' est ',
    {
        type: 'select',
        valueName: 'GENDER',
        options: {
            female: 'allée',
            other: 'allé'
        }
    },
    ' à ',
    '${CITY}',
    '.'
]
```

This doesn't have to happen inside the tokenizer so feel free to change things.