constraints.txt highlighting

[WaleedSehgal]

Fixes #1053

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[msftclas]

All CLA requirements met.

[DonJayamanne]

Change to 'Add syntax highlighting to constraints.txt file to match that of piprequirements files'

[DonJayamanne]

Move this file into 3 Code Health

[codecov]

Codecov Report

Merging #1685 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1685      +/-   ##
==========================================
+ Coverage   71.38%   71.53%   +0.15%     
==========================================
  Files         277      277              
  Lines       13014    13014              
  Branches     2344     2344              
==========================================
+ Hits         9290     9310      +20     
+ Misses       3583     3569      -14     
+ Partials      141      135       -6

Impacted Files	Coverage Δ
src/client/debugger/mainV2.ts	77.82% <0%> (+2.82%)	⬆️
src/client/debugger/PythonProcess.ts	51.25% <0%> (+5.41%)	⬆️

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[brettcannon]

You should also support files simply named constraints.txt (which requires simply adding that to "filenames" above.