This uses https://domluna.github.io/JuliaFormatter.jl/stable/ with a
format.jl script wrapper to handle line ranges and input from stdin.

This formatter uses a script wrapping the [JuliaFormatter.jl library](https://github.com/domluna/JuliaFormatter.jl).  This "formatter script in the plugin" approach hasn't been taken before in `vim-codefmt`, so let me know if you'd prefer a different approach or to reorganize things.  (The JuliaFormatter.jl library ships with an executable script, but it only works on file paths (not stdin) and can't handle line ranges.  I'm also not sure how to get the path of a bin directory in your Julia environment, which doesn't work like executables in a package management system like npm.)

If the user hasn't installed the `JuliaFormatter` package, instructions will point to an `install` script in the same directory as the formatter script.  I've tested the shell version of this script, but don't have a Windows machine to test the `.cmd` version.

The line-range handling is a bit of a hack: the underlying library doesn't expose this functionality, so I used comments to enable/disable the formatter around relevant blocks.  I found some edge case behaviors that I've hopefully documented clearly.

The script will use the same `.JuliaFormatter.toml` files in the parent directory hierarchy of the source file being edited to determine the formatter options like style, indent, and line width.  This is the same behavior that running the executable from JuliaFormatter.

Fixes #231.

Also sort lists, document which filetypes support Prettier, and match
the AutoFormatBuffer example to the language list.

Forgot to pass the "-" placeholder for stdin. Apparently if you pass
--stdin-filename ruff assumes stdin, but if there's no filename it can
get confused and format everything in-place instead of formatting the
editor buffer.