Nest() function

[nblumhardt]

Serilog 4.2 stabilized the use of dotted.identifiers in message templates. Properties captured using this mechanism have dotted names, so when {a.b} is captured in a log statement, the resulting property is called "a.b", it doesn't become an object a with sub-object b.

The Nest() function in this PR implements the conversion from dotted names to nested objects so that dotted identifiers can effectively capture fully-structured data, for back-ends/output formats that support it.

Given the event:

Log.Information("HTTP {request.method} {request.path}: {response.status_code}", "GET", "/example", 200);

And the Serilog.Expressions template:

    .WriteTo.Console(new ExpressionTemplate("{ {@t, @m, @l, @x, ..Nest(@p)} }\n"))

The resulting JSON output will be:

{"@t":"2025-02-27T12:28:17.3004399+10:00","@m":"HTTP GET /example: 200","@l":"Information",
"request":{"method":"GET","path":"/example"},"response":{"status_code":200}}

A separate commit is included which updates the project to match the newer Actions-based builds.

[nblumhardt]

Build failure is expected because of the switch to Actions.