Formatting fails when one justfile imports another#76

If one justfile imports another, like `import "../justfile"`, then copying the justfile we're editing to a temporary directory and running `just --fmt --unstable` doesn't work. This is the current approach taken by this project [here](https://github.com/terror/just-lsp/blob/master/src/server.rs#L497-L520). Instead, we can run `just --dump --justfile {filename}"` with the full path to the file. This also allows for non-standard filenames like `something_else.just`.