Refactor/replace URI utilities with dedicated type

[bollwyvl]

Elevator Pitch

We should

• accept a dependency on an external URI library
• get one into JupyterLab/lumino
• write our own
  To handle various URI-related tasks.

Motivation

#403 (with work on #406) demonstrated how easy it is to mess up URIs vs ContentsManager paths (which are not URI-encoded). We have a fair amount of places where we do string manipulation on URIs, which is almost never safe.

Design Ideas

• https://github.com/microsoft/vscode-uri is already a dependency, by way of the some of the upstream stuff we use... it may not be getting shipped to the browser, but very probably is
  • [wip] Add Language Server Proxy Kernel #278 has some commits somewhere where I tried this, but I ran into some issues
• writing our own (or subclassing) might let us put more interesting behavior in it, like transforming to ContentsManager paths