Initial setup of .devcontainers for Codespaces

[MichaelChirico]

I recently set this up for data.table to do remote dev, it's quite convenient for working with VScode-in-the-browser offered by GitHub to actually run the code instead of just typing it+relying on CI.

Ref: https://docs.github.com/en/codespaces/setting-up-your-project-for-codespaces/adding-a-dev-container-configuration/introduction-to-dev-containers

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e1862b2) 98.68% compared to head (2e1528e) 98.68%.

❗ Current head 2e1528e differs from pull request most recent head 7d10aed. Consider uploading reports for the commit 7d10aed to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2403   +/-   ##
=======================================
  Coverage   98.68%   98.68%           
=======================================
  Files         126      126           
  Lines        5698     5698           
=======================================
  Hits         5623     5623           
  Misses         75       75           

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[IndrajeetPatil]

Cool!
Will give this a try myself.