Explore how to implement isIncomplete support

[krassowski]

References

Partially fixes #776 but does not work reliably; instead an implementation upstream in JupyterLab is needed. We need to think how to add this. Supporting isIncomplete is needed because:

• it enables faster completions when latency between server is low
• it is a non-optional part of LSP spec
• servers like TexLab and R server depend on it for completions

Not sure if i will be merging this one (not happy with how non-reliably this works), unless I can fix it without relying on extensive changes upstream.

Also upstream: reply type for completion should be AnyCompletion | null.

TODO:

• use TriggerForIncompleteCompletions
• fix update issues

Code changes

User-facing changes

Backwards-incompatible changes

Chores

• linted
• tested
• documented
• changelog entry

[github-actions]

👈 Launch a binder notebook on branch krassowski/jupyterlab-lsp/fix-incomplete-completion