Adds a structured logger with output to logfmt format (which is both human readable and supported by most log ingestors).

Some ideas borrowed from https://github.com/karlseguin/log.zig.

My goal is to give each connection a unique conn_id, and each CDP message a req_id. The logger will get passed into CDP + Browser/Session/Page and even ideally into zig-js-runtime. The goal is to generate logs like:

_time=1739795092929 _level=info _ctx=client_connected conn_id=339494
_time=1739795092392 _level=info _ctx=http_request conn_id=339494 path=/json/version status=200
_time=1739795093001 _level=info _ctx=http_request conn_id=339494 path=/
_time=1739795093450 _level=info _ctx=cdp_request conn_id=339494 req_id=1 method=CSS.enable
...

Every log has a _time, _level and _ctx field. The "context" is just meant to be a unique label - nothing worse than getting a log message and not knowing where it originated (although it isn't a substitute for metrics, group by on _ctx can also be used to detect issues).

If everyone's ok with this, I want to close this PR and re-open it after #422 has been merged with more of the std.log switched over to use this.