chore: improve constants

[Rich-Harris]

This started out as a minor tweak, to use consistent casing for constants, but I soon realised there's a bunch of improvements we can make:

• we have two constants.js files, that's confusing (even if the intent was to have one that could only be used by compiler code, we weren't doing that consistently)
• duplication — we don't need both PassiveEvents and PassiveDelegatedEvents
• more helpers (for example the new is_rune helper gives us type narrowing without forcing ugly casting on the caller side)
• some things are only used in one module, and should live in that module

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