Add MonacoEditor.jl

[terasakisatoshi]

Hello, @fonsp . As we discussed in the aforementioned pull request on GitHub (AtelierArith/PlutoMonacoEditor.jl#1), I am making a new pull request for this repository. This new pull request adds the file src/web/MonacoEditor.jl, which demonstrates how to implement a rich text editor using Monaco Editor inside Pluto.

[terasakisatoshi]

What we need is adding a frontmatter something like:

#> [frontmatter]
#> license_url = "https://github.com/JuliaPluto/featured/blob/2a6a9664e5428b37abe4957c1dca0994f4a8b7fd/LICENSES/Unlicense"
#> image = "<I'm not sure what image shoud be added>"
#> order = "<Inputme>"
#> tags = ["javascript", "web", "classic"]
#> license = "Unlicense"
#> description = "<Could you help me?>"
#> 
#>     [[frontmatter.author]]
#>     name = "Pluto.jl"
#>     url = "https://github.com/JuliaPluto"