Forward pluto_* args verbatim to launch parameters

[fonsp]

Summary

Redesigns the @pluto block argument syntax. The only recognised special key is now notebook (local .jl file to run). Any other key must start with pluto_ and is forwarded verbatim to the frontend as window.<key> = <value>; in the launch-parameters script.

This removes the hand-picked set of named fields (url, state, binder, disable_ui) and lets users reach any Pluto frontend global without DocumenterPluto growing a field for each one.

Before

```@pluto
url        = "https://example.com/n.jl"
state      = "https://example.com/n.plutostate"
binder     = "https://mybinder.org/…"
disable_ui = false

### After

pluto_notebookfile = "https://example.com/n.jl"
pluto_statefile    = "https://example.com/n.plutostate"
pluto_binder_url   = "https://mybinder.org/…"
pluto_disable_ui   = false

`notebook = "…"` still works for local notebooks; the builder resolves it and emits `pluto_notebookfile`/`pluto_statefile` internally. `pluto_disable_ui` defaults to `true` (unchanged behaviour for docs builds).

## What changed

- `PlutoBlock` now stores `notebook::Union{String,Nothing}` and `pluto_params::Dict{String,Any}` instead of one field per attribute.
- Parser rejects any top-level key that isn't `notebook` or a `pluto_`-prefixed key.
- Validation: either `notebook` is set, or both `pluto_notebookfile` and `pluto_statefile` are set — not both.
- Domify emits every entry of `pluto_params` as `window.<key> = <js>;`. Local-notebook paths overlay `pluto_notebookfile`/`pluto_statefile` via `relhref`. Added a small `_js_value` helper that handles String / Bool / Number / Vector / Dict / nothing.
- Builder loses its pre-hosted-URL branch — for remote notebooks, everything is already sitting in `pluto_params`.

## Test plan

- [x] `Pkg.test()` — all 12 parsing tests pass; the end-to-end test errors on a pre-existing `RawHTMLHeadContent` / `HTMLAsset` version mismatch that also fails on `main`.
- [ ] Spot-check a real docs build with a local notebook and a mix of `pluto_*` overrides.

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