add overrides.d for settings defaults

[bollwyvl]

Elevator Pitch

Allow extensions to package {app_settings_dir}/overrides.d/*.json files to be merged together with schema defaults.

This is relevant for third-party extensions that would benefit from being able to be gently configured by other extensions:

• jupyter-lsp: suggested configuration for one or more language_servers
• jupyterlab-tour: a user tour
  • this may well not work, as they are lists, which have undefined merge characteristics...

References

• fixes Adopt a distribution-friendly conf.d-style settings defaults configuration #223

Changes

• adds a conf.d-style loader for overrides.d/*.{json,json5}
  • these are read before the singleton json
• as it's "more human," json5 always wins
• ensures merging occurs at the extension level

[codecov-commenter]

Codecov Report

Merging #224 (a50ce09) into master (d577120) will increase coverage by 0.47%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master     #224      +/-   ##
==========================================
+ Coverage   76.96%   77.44%   +0.47%     
==========================================
  Files          28       28              
  Lines        2071     2097      +26     
==========================================
+ Hits         1594     1624      +30     
+ Misses        477      473       -4     

Impacted Files	Coverage Δ
jupyterlab_server/settings_utils.py	86.04% <92.85%> (+2.47%)	⬆️
jupyterlab_server/tests/test_settings_api.py	100.00% <100.00%> (ø)

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[blink1073]

Thank you!