Automatically activate syntax highlighting using firstLine in package.json to resolve issue 3892

[cgahlon]

For #

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• package-lock.json has been regenerated by running npm install (if dependencies have changed)

[msftclas]

All CLA requirements met.

[codecov]

Codecov Report

Merging #3893 into master will decrease coverage by 3%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #3893    +/-   ##
=======================================
- Coverage      81%     79%    -2%     
=======================================
  Files         399     407     +8     
  Lines       18285   18610   +325     
  Branches     2951    2996    +45     
=======================================
- Hits        14719   14583   -136     
- Misses       3563    4024   +461     
  Partials        3       3

Flag	Coverage Δ
#Linux	68% <ø> (+1%)	⬆️
#Windows	68% <ø> (+1%)	⬆️
#macOS	68% <ø> (+1%)	⬆️

Impacted Files	Coverage Δ
...r/debugAdapter/DebugServers/RemoteDebugServerv2.ts	22% <0%> (-57%)	⬇️
...er/debugAdapter/DebugServers/LocalDebugServerV2.ts	27% <0%> (-56%)	⬇️
src/client/linters/pydocstyle.ts	39% <0%> (-54%)	⬇️
...debugger/debugAdapter/DebugClients/DebugFactory.ts	47% <0%> (-53%)	⬇️
src/client/datascience/jupyter/jupyterServer.ts	23% <0%> (-49%)	⬇️
...ugger/debugAdapter/DebugServers/BaseDebugServer.ts	24% <0%> (-41%)	⬇️
src/client/datascience/historyProvider.ts	53% <0%> (-39%)	⬇️
src/client/datascience/history.ts	25% <0%> (-36%)	⬇️
src/client/datascience/jupyter/jupyterExporter.ts	49% <0%> (-36%)	⬇️
src/client/datascience/statusProvider.ts	35% <0%> (-33%)	⬇️
... and 137 more

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[DonJayamanne]

@cgahlon
Thanks for the PR, however we only accept PRs that resolve an existing issue.
Please could you file an issue on GitHub the issue you are attempting to resolve.

[cgahlon]

@DonJayamanne - I referenced this pull request in #3892

This is my first PR on github so let me know if there is another step to follow.