Attach `TextDocumentEdit[]` to completion item

Right now, the additionalTextEdits field on CompletionItem only allows providing a list of TextEdits that accepting the completion will add.

We have a use case where accepting a completion item needs to modify a separate file, not the file where the user is currently typing. TextEdit can only change the current file, while a TextDocumentEdit can change any file.

The use case is vaguely registering that the current "package" depends on another package. Some examples:

• Concretely: I am working on Sorbet, a Ruby language server. Sorbet provides a custom notion of packages to encourage developers to be explicit about dependencies and module boundaries in a large Ruby monorepo. Package dependencies are specified in separate files, not using import statements at the top of the file.
• Another idea: an LSP server wants to automatically add the relevant npm package to a user's package.json file which provides an import (or a relevant RubyGems package to a user's Gemfile)
• Another: accepting a completion item changes a Bazel build file target to allow a dependency to be used from the current file.

etc. etc.

I have yet to begin an implementation, but our current recourse is to use the Command associated with a completion item to make these edits. Using Commands for this purpose is particularly heavy-weight, though, and it would be much easier if we could find a way to pass TextDocumentEdits back to the client directly.

Start with CompletionItem.additionalTextEdits and the TextEdit/TextDocumentEdit definitions described in the issue. Compare the requested cross-file completion behavior with the existing Command workaround and define a protocol shape that lets clients apply edits to another file. Done means the proposal specifies the behavior and its compatibility implications.