GH1547: Fix issue with parsing escaped curly braces in FormatParser

[kcamp]

This addresses the issues described in GH-1547 regarding escaping curlies.

As detailed in the original issue, testing this fix uncovered an additional bug (inconsistency with string.Format implementation)

The one thing that I think this exposes is a bug in the LiteralToken that is being returned for escaped curlies
Information("{{}} {0}", 123); outputs "{{}} 123"
@DixonDs is right in that this should probably output {} 123 to be consistent with expectations, namely the implementation of string.Format. I have adjusted the parser to return a single '{' or '}' in those cases and updated the tests to match.

[devlead]

LGTM! 👍

[devlead]

@kcamp your changes have been merged, thanks for your contribution 👍