Experimental JS API for getting/setting cell metadata

[fonsp]

We already had epxeirmental JS API for notebook metadata, but now also for cells!

Example notebook:

### A Pluto.jl notebook ###
# v0.19.27

using Markdown
using InteractiveUtils

# ╔═╡ 339c3ba2-95e1-44a2-9601-8382e0cfb9cf
# ╠═╡ number = 0.13315567181932053
html"""
<script>
console.log("before", getCellMetadataExperimental("number"))
await setCellMetadataExperimental("number", Math.random())
	
console.log("after", getCellMetadataExperimental("number"))

// await deleteCellMetadataExperimental("number")
</script>
"""

# ╔═╡ 9f26c140-24b0-11ee-1332-c3ced47002e2
# ╠═╡ aa = 1232
html"""
<script>
await setCellMetadataExperimental("aa", 777, {cell_id: "f8fc0eb8-0ef6-4096-a54f-c04c741f8258"})
console.log(getCellMetadataExperimental("aa", {cell_id: "f8fc0eb8-0ef6-4096-a54f-c04c741f8258"}))
await deleteCellMetadataExperimental("aa", {cell_id: "f8fc0eb8-0ef6-4096-a54f-c04c741f8258"})
</script>
"""

# ╔═╡ f8fc0eb8-0ef6-4096-a54f-c04c741f8258


# ╔═╡ Cell order:
# ╠═339c3ba2-95e1-44a2-9601-8382e0cfb9cf
# ╠═9f26c140-24b0-11ee-1332-c3ced47002e2
# ╠═f8fc0eb8-0ef6-4096-a54f-c04c741f8258

cc @lucaferranti

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="secret-JS-API-for-cell-metadata")
julia> using Pluto