Lsp rename,delete and create file

[akinsho]

Lsp rename,delete and create file

[amitkot]

This VSCode issue might contain relevant pointers for dart's lsp support of relevant APIs.

[akinsho]

@amitkot this is already implemented via the LSP it turned out I didn't need to add much, it's a setting in the language server settings renameFilesWithClasses = 'prompt' if you add this that functionality should already work.

I added and tested it a while ago and have been using it. Just forgot to update this.
The behaviour is documented in https://github.com/dart-lang/sdk/blob/master/pkg/analysis_server/tool/lsp_spec/README.md#client-workspace-configuration

The deletion and creation part aren't really necessary since the LSP client in nvim communicates with the LSP about what files are created or deleted, so that doesn't need to be here or need special handling. I'm going to actually close this out, since I just forgot to do it.

I've just added the options to the docs so it's clearer that it's available