Test templates by comparing output with existing formatters

[nblumhardt]

I'm frequently pointing people to this library as a means to customize JSON output, with CompactJsonFormatter, RenderedCompactJsonFormatter, or JsonFormatter as the starting point.

This PR adds a test that attempts to replicate these formats byte-for-byte, so that we can identify gaps and inconsistencies.

Along the way:

• Added @i and @r properties that mirror the CLEF built-ins with those names
• Added UtcDateTime(d), Now(), ToString(x, f) (fixes [Question] Timestamp format on json template #18)
• Fixed wildcard comprehensions with unary conditions - IsDefined(@p[?]) now works as expected (any property is defined :-))
• Spotted some instances of Make the specified format provider (culture) available to runtime operations #19 and added comments
• Re-targeted tests to net5.0
• Bumped version to 1.1

[nblumhardt]

🤔 PR isn't triggering CI; could be because I inadvertently PR'd from the repo itself? Going to live dangerously and see if I can get a build out of dev :-)