refactor(libutil): convert Verbosity to enum class

[lovesegfault]

Motivation

Convert the Verbosity enum from old-style typedef enum to modern
enum class with cleaner value names:

• lvlError -> Verbosity::Error
• lvlWarn -> Verbosity::Warn
• lvlNotice -> Verbosity::Notice
• lvlInfo -> Verbosity::Info
• lvlTalkative -> Verbosity::Talkative
• lvlChatty -> Verbosity::Chatty
• lvlDebug -> Verbosity::Debug
• lvlVomit -> Verbosity::Vomit

I also added ++/-- operators with clamping for verbosity adjustments, and
explicit casts where Verbosity is serialized to integers.

Context

Part-Of: #14658

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[xokdvium]

Don't think this is a good idea, since it doesn't bring any practical benefits. There's nothing fundamentally wrong with plain enums other than namespacing and the occasional implicit conversion. New types can surely use enum classes, but this churn is too annoying to deal with for backports.

Btw the diff could be reduced by bringing the names into the global scope. This will provide the benefit of banning implicit conversions to ints without the churn:

using enum Verbosity;

[Ericson2314]

Btw the diff could be reduced by bringing the names into the global scope. This will provide the benefit of banning implicit conversions to ints without the churn.

To be clear @xokdvium likes this, and I do too. I would be fine as is — we should definitely get enum struct. Doing it this was assuages the churn concerns while still getting us the main benefits.