Fix signature blur and fix formatting when no arguments are present

[krassowski]

References

Follow up after #671:

• signature (and details) should be clickable but were not due to incorrect use of target where relatedTarget should be used
• label code was not rendering in signature for functions without arguments if details were present due to a missing new line

Code changes

• use relatedTarget in blur event
• always add two new lines if details are, not only when highlighting active arguments

User-facing changes

Before:

After:

Backwards-incompatible changes

None

Chores

• linted
• tested
• documented
• changelog entry

[github-actions]

👈 Launch a binder notebook on branch jupyter-lsp/jupyterlab-lsp/signature-details