chore: refactor $inspect

[Rich-Harris]

This is a precursor to a more useful $inspect rune (or more accurately, a more useful $state.snapshot rune). It's mostly just moving some code around (making runtime.js less hilariously overstuffed), but also provides a better error when $inspect is used in the wrong place. (Side-note: the error we print when using $effect or $effect.pre outside an effect is wrong — it says you can only use them during component initialisation which is obviously untrue. Will open a PR to fix.)

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

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[Rich-Harris]

self-merging to unblock other work