Add some progress bars for time-intensive tasks

[srawlins]

It's a lot of code to get this to display nicely but here goes:

• Refactor logging.html into a class structure to store a few fields; the basic behavior is kept in-tact.
• Privatize progressLevel.
• Add top-level functions for setting up, completing, and making progress on a progress bar.
• Remove usage of cli_util, which provides it's own Logger class; we were only using it to detect ANSI support, and that can be done inline.
• Add a deterministic progress bar for when elements are being discovered and linked. Also for when docs are being precached. The precaching is necessary so that we can do global discovery of {@template}s. 😦 .
• Change _allModelElements from a late final field with a self-executing closure to a function. I realized this "field" was only being used once, and it can take minutes and minutes to initialize this "field" when documenting Flutter. 😛 . So... call it a function.
• The ProgressBar class comes from package:nnbd_migration.
• The task code is really awkwardly complex. But I think I added comments to help. I tried to simplify the commands to document a package, and then added a "To re-run this..." output line because all of the pre-work that the task does can take minutes. The awkwardness is mostly from the --json flag which is needed when doing things like comparing warnings, but cannot be used when you're trying to watch progress.

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[kallentu]

Can we add a comment here? For also why it's set to 502?

[srawlins]

Good catch. Added. The number, is not important; it's just a unique Level for the logging code.