[utils] Ban enums, and replace them with alternatives

Nobody has claimed this yet.

Carried over from https://github.com/MetaMask/utils/pull/280, which was approved but still open when `@metamask/utils` moved into core.

TypeScript is structurally typed nearly everywhere, but enums are nominal. Two enums that look identical are not assignable to each other, so a value produced by one copy of a package fails to typecheck against a function from another copy. That bites hardest in a monorepo with duplicated versions in the tree.

`packages/utils` still exports three:

- `KnownCaipNamespace` in `src/caip-types.ts:134`
- `JsonSize` in `src/misc.ts:137`
- `Duration` in `src/time.ts:4`

The original PR replaces them with const objects plus derived union types, which keeps the call sites working while making the types structural.

Note this is a breaking change for consumers who use the enum types nominally, so it wants a major. The PR's `.eslintrc.js` rule does not port directly since core has its own shared config.

Start in packages/utils at src/caip-types.ts:134, src/misc.ts:137, and src/time.ts:4 to inspect the three exported enums and their call sites. Replace them with const objects and derived union types while preserving call-site behavior, and adapt the lint rule to core's shared configuration. Done means the enum exports are structural and the breaking change is prepared for a major release.

Written by the indexing model from the issue text.