Adding format=symfony for imports

[timwhitlock]

This is required for XLIFF 2.0 files produced by Symfony's XliffFileDumper.

The name attribute is only rendered when the translation key is 80 bytes or less.

When absent, Loco's standard XLIFF parser would fall back to the id attribute in order to establish the unique translation key. The format=symfony parameter overrides this behaviour, ensuring that the key is taken from the <source> element as per Symfony's use of XLIFF.