child_process: add tracing channel for spawn

[marcopiraccini]

This PR adds a new child_process.spawn tracing channel wrapping the spawn call. Since spawn is synchronous, this uses start, end, and error events (rather than the async variants).

Events

• tracing:child_process.spawn:start — published before the native _handle.spawn() call, with { process, options }
• tracing:child_process.spawn:end — published on successful spawn, with { process }
• tracing:child_process.spawn:error — published on spawn failure, with { process, error }

Since exec, execFile, and fork all call spawn internally, this single tracing channel covers all child process creation paths (e.g.: https://github.com/nodejs/node/blob/main/lib/child_process.js#L174)