[wip] support --json flag for pages publish

[tjenkinson]

Fixes #2437

What this PR solves / how to test:

Adds a --json flag to pages publish command, that outputs the URL to the deployment.

This is still WIP but would be great to get some feedback on the approach here. The idea is to put the logger into a mode where it will not use stdout, and then any log messages that would have gone to stdout will be redirected to stderr instead, just leaving stdout for the json.

• I wonder if the logger should become instance based and passed through to things that need it?
• Or should we have a global --json flag that constructs the logger in a mode with stdout disabled to prevent any chance of something getting through before it's configured?
• Should we stop using stdout and reserve that just for machine readable messages?

I noticed there's one other place already that already respects a --json flag in the cli.

Associated docs issue(s)/PR(s):

• [insert associated docs issue(s)/PR(s)]

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[penalosa]

Thanks for this PR! Apologies for the delay here, but in the meantime we implemented a different way to do what this PR proposes. Setting the WRANGLER_OUTPUT_FILE_PATH env var to a file path will trigger Wrangler to write JSON-NL metadata about what it's doing to that file, including the Pages deployment URL. Because of that I'm going to close this PR for now, but feel free to open a new issue if that doesn't fully address your use-case.