Code formatter

[fonsp]

Integrates JuliaFormatter.jl into Pluto:

TODO:

• Format on save, remove button
• Revisit 📂 Auto-save when the Expr did not change #1055 : don't re-run cells with whitespace changes
• Persist cursor position after format?
• Format while editing?
• Can we indent with tabs rather than spaces? domluna/JuliaFormatter.jl#421 to keep consistency with old notebooks
• Decide if the formatter is good enough to always enable, maybe disable some features

I just want to note that code formatting is one of those things that people-with-a-github-account can care passionately about, but keep in mind that our intended audience is beginner programmers. The hope of adding a formatter is to remove 'accidental complexity' that comes from "weirdly formatted code" that you can get from copy-pasting snippets, and to make everyone's code easier to read. (Also: we think that a code editor should let you write an AST-with-comments, not arbitrary text. Whitespace is your code's presentation, just like syntax highlighting.)