Move from yarn to npm

[brettcannon]

Fixes #1402

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)

[brettcannon]

So Travis says it's passing, but if you look at the details every single test failed. https://travis-ci.org/Microsoft/vscode-python/jobs/385379858#L677 suggests there might be a problem. Do you happen to know what's going on, @DonJayamanne ?

[codecov]

Codecov Report

Merging #1771 into master will decrease coverage by 0.38%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1771      +/-   ##
==========================================
- Coverage   74.44%   74.05%   -0.39%     
==========================================
  Files         282      282              
  Lines       13253    13089     -164     
  Branches     2382     2335      -47     
==========================================
- Hits         9866     9693     -173     
- Misses       3257     3264       +7     
- Partials      130      132       +2

Impacted Files	Coverage Δ
src/client/providers/symbolProvider.ts	23.33% <0%> (-59.36%)	⬇️
src/client/language/braceCounter.ts	81.08% <0%> (-13.52%)	⬇️
src/client/common/variables/environment.ts	92.45% <0%> (-4.92%)	⬇️
...lient/debugger/configProviders/pythonV2Provider.ts	83.33% <0%> (-4.01%)	⬇️
...reter/locators/services/cacheableLocatorService.ts	91.83% <0%> (-2.05%)	⬇️
src/client/interpreter/locators/index.ts	90% <0%> (-1.49%)	⬇️
...rc/client/debugger/PythonProcessCallbackHandler.ts	53.61% <0%> (-0.66%)	⬇️
src/client/debugger/Main.ts	51.85% <0%> (-0.5%)	⬇️
src/client/common/configSettings.ts	88.09% <0%> (-0.35%)	⬇️
src/client/language/tokenizer.ts	97.79% <0%> (-0.23%)	⬇️
... and 16 more

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[DonJayamanne]

@brettcannon
Found the problem, temporary solution is to change "@types/node": "^9.4.7", to "@types/node": "9.4.7",

We'll need to update the dev and standard dependencies together next time.

Here's a detailed explanation of the problem:

• Iconv-lite relies on node.js (< 9.6)
• Node.js typings are in dev dependencies (>= 9.4.7)
• Package.lock is pulling down node.js 9.6.*
• The new dev version of node.js typings doesn't match with iconv-lite

[d3r3kk]

Pretty much exactly what I figured this change would entail.

[d3r3kk]

If Travis succeeds then this PR is safe to go in, I'll update the CI on VSTS to switch over to npm now.

NOTE: I killed the VSTS build, please ignore it for the purpose of this PR.

Edit: Claim responsibility for CI build cancellation.