Add semantics for @ and $

[Stephen Cleary]

Currently the specification calls out @ and $ prefixes, but their semantics are not specified.

It would be nice to have at least an indication of how they are commonly used, even if the specifics are implementation-dependent.

[Robin Sue]

In Serilog, they're defined as follow:

The @ operator in front of SensorInput instructs Serilog to preserve the structure of the object passed in. If this is omitted, Serilog recognises simple types like strings, numbers, dates and times, dictionaries and enumerables; all other objects are converted into strings using ToString(). 'Stringification' can be forced using the $ operator in place of @.

[Nicholas Blumhardt]

👍 - we should probably insert that passage directly into the spec.