Fix handling null workspace folder and untitled document

[renkun-ken]

Closes #460

This PR makes handling null workspace folder and untitled document with the following logic:

1. If the workspace folder is not null, then use it as the root path.
2. Otherwise, if path from uri is not empty, use the directory of the path.
3. Otherwise, the uri is not a file:// uri, then use the working directory of the language server process.

[renkun-ken]

Currently, our test util functions do not directly support working with untitled documents, as they only accept path instead of uri. Should we generalize the respond_* functions so that they could also work with uri, or we directly use respond() in the tests for untitled documents which is only a tiny portion of the test cases, maybe not worth making any changes to specific respond_* functions.

[randy3k]

I think it should not be too difficult to support it. By the way, what will be the uri for untitled files? (Too lazy to check)

[renkun-ken]

Uri for untitled document is in the form of untitled:Untitled-1.

Also, there are other potential uri format like vscode-notebook-cell://home/ken/test.Rmd#000001 to represent a VS Code notebook cell in a notebook editor.

[renkun-ken]

I updated the test util functions by supporting uri input directly.

[randy3k]

LGTM.