Notebook metadata

[ctrekker]

This PR adds a field metadata to Pluto's notebook structure. Nothing too special, but a few details are worth mentioning:

• Metadata is saved directly below file version number and above first cell
• Updates to metadata from client is synced over WS properly

TODO

• Save / load mechanism
• Client synchronization
• Tests
• Read-only testing to pin format not necessary

Prefix Ideas

This is arguably the least important part of this PR, but currently the metadata is saved in the following format:

### A Pluto.jl notebook ###
# v0.18.4

#> [frontmatter]
#> title = "Fancy title!"
#> description = "......"
#> other_metadata = 42

...rest of notebook here...

This format works just fine, but Pluto's cell prefix format sets a precedent for wild and unique prefixes. If anyone has ideas that would make it implicitly clear that the lines marked with this prefix are notebook-level metadata statements, let me know.

A box around properties could be neat (but maybe overkill or distracting and maybe make editing properties from the file mildly annoying)

### A Pluto.jl notebook ###
# v0.18.4
# ┌────────────────────────┐
# │ title = "Fancy title!" │
# │ description = "......" │
# │ other_metadata = 42    │
# └────────────────────────┘

...rest of notebook here...

[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="cb/notebook-metadata")
julia> using Pluto

[fonsp]

I'm a bit worried about the limitations of TOML, specifically mixing Dicts and other types in a vector:

julia> TOML.print(Dict("hello" => [Dict("a" => "b"), "c"]))
[[hello]]
a = "b"
[[hello]]
ERROR: array should contain only tables
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] _print(f::Nothing, io::Base.TTY, a::Dict{String, Vector{Any}}, ks::Vector{String}; indent::Int64, first_block::Bool, sorted::Bool, by::Function)
   @ TOML.Internals.Printer /Applications/Julia-1.7 x86.app/Contents/Resources/julia/share/julia/stdlib/v1.7/TOML/src/print.jl:160
 [3] #print#13
   @ /Applications/Julia-1.7 x86.app/Contents/Resources/julia/share/julia/stdlib/v1.7/TOML/src/print.jl:168 [inlined]
 [4] print(a::Dict{String, Vector{Any}}; sorted::Bool, by::Function)
   @ TOML.Internals.Printer /Applications/Julia-1.7 x86.app/Contents/Resources/julia/share/julia/stdlib/v1.7/TOML/src/print.jl:171
 [5] print(a::Dict{String, Vector{Any}})
   @ TOML.Internals.Printer /Applications/Julia-1.7 x86.app/Contents/Resources/julia/share/julia/stdlib/v1.7/TOML/src/print.jl:171
 [6] top-level scope
   @ REPL[9]:1

which can even lead to a TOML.jl bug creating an invalid file: JuliaLang/julia#45340

EDIT: well we already made this choice for cell metadata, so let's just be consistent...

[fonsp]