refactor(treewide): modern C++ idioms

[lovesegfault]

Motivation

This is a tracking PR for my work around C++ modernization. It should not be reviewed, changes will be fished out into individual PRs.

Context

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[edolstra]

Some of these are unnecessarily code-churny with little benefit (especially the enum stuff, i.e. lvlError -> Verbosity::Error, tInt -> InternalType::Int). The merge conflicts they cause with open PRs/branches (441+) negate any potential benefit.

[Ericson2314]

I do like using enum struct because the standard allows less verbose switch than regular enums (no need for default always, I think?), and also far fewer implicit conversations.

I don't really think the merge conflict argument against churn holds water anymore because the LLMs will cruise through trivial merge conflicts from changes like that l.