Add Formatting plugin

This is a new plugin that can format code in two ways:

• Automatic formatting on save
• Via a single exposed action named "Format Document"

Supported formatters

• clang-format
• prettier
• dartfmt
• rustfmt
• xmllint for xml
• jq for json
• go fmt

Compared to 'External Tools', this is a lot nicer to use in my opinion because

• it provides a single interface to do formatting
• it applies formatting as a patch on top of the document which helps retaining the undo history and provides an easy way to undo the changes quickly.
• has more control, for e.g., with clang-format it is able to format only the changed lines of a document and not the whole document

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Feedback welcome.