Implement jump target selector and jump to references

[krassowski]

References

Fixes #1. Closes #18 as outdated.

Code changes

• fix some incorrect types for Hover and references client requests
• add target selections if multiple choices exist for jump target
• allow jumping to references
• remove unused "Highlight references" and "Highlight type references" commands and context menu entries which were doing nothing
• deprecate internals manual getters and protocol invokations in ws connector (mostly for myself so I see their uses and can remove them easily)

TODO robot tests for:

• test jumping to node_modules/@jupyterlab/ to make sure decoding (of @) is handled correctly
• test jump target selection
• test references
• update README

User-facing changes

• Alt + 🖱️ click will now jump to references if the initial jump to definition indicates that user's clicked on the (only) definition, or if no definition was found
• jumping to sources in packages with namespaces (@) will now work correctly
• when multiple jump targets are present, a selection dialog will show up

Backwards-incompatible changes

• Jump to definition feature API got restricted and refactored

Chores

• linted
• tested
• documented
• changelog entry

[github-actions]

👈 Launch a binder notebook on branch jupyter-lsp/jupyterlab-lsp/jump-to-references