Add CodeQL analysis to GitHub workflow

[pellared]

Add GitHub's code scanning.
More info:

• https://github.blog/2020-09-30-code-scanning-is-now-available/
• https://docs.github.com/en/github/finding-security-vulnerabilities-and-errors-in-your-code/about-code-scanning
• https://securitylab.github.com/tools/codeql

[codecov]

Codecov Report

Merging #66 (4cc9e89) into master (a4b9284) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #66   +/-   ##
=======================================
  Coverage   50.00%   50.00%           
=======================================
  Files           1        1           
  Lines           2        2           
=======================================
  Hits            1        1           
  Misses          1        1           

Flag	Coverage Δ
Linux	50.00% <ø> (ø)
Windows	50.00% <ø> (ø)
macOS	50.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a4b9284...4cc9e89. Read the comment docs.