Improve: kernel completions in R, completions in strings and paths

[krassowski]

References

Fixes #667 and fixes #639.

Code changes

• fix kernel completions not showing up for R due to lettercase difference in identifiers
• fix tab completions not showing up in strings due to incomplete trigger kind invalidation
• fix path completions reconciliation for pyls/pylsp with IPython by working around different prefixes returned by LSP and kernel as a special case

User-facing changes

Backwards-incompatible changes

None

Chores

• linted
• tested
• documented
• changelog entry

[github-actions]

👈 Launch a binder notebook on branch jupyter-lsp/jupyterlab-lsp/completion-fixes