Add support for folding of docstrings and comments

[Don Jayamanne (DonJayamanne)]

fixes #893

Unit tests will fail, as the API is not yet available in standard build of VS Code.

Either way, please review.

[Mikhail Arkhipov (MikhailArkhipov)]

I'd also add basic tests - empty file, unclosed string, odd sequences like """ s1 """ """ s2 """ without like breaks

[Don Jayamanne (DonJayamanne)]

Easy, will do.
I think unclosed strings will be treated as strings by tokenizer, wouldn't it.

[Mikhail Arkhipov (MikhailArkhipov)]

Yes, it is more how folding code calculates 'next' line position when there may be none

[codecov]

Codecov Report

Merging #894 into master will decrease coverage by 17.54%.
The diff coverage is 55.33%.

@@            Coverage Diff             @@
##           master    #894       +/-   ##
==========================================
- Coverage   71.65%   54.1%   -17.55%     
==========================================
  Files         273     275        +2     
  Lines       12700   12745       +45     
  Branches     2282    2288        +6     
==========================================
- Hits         9100    6896     -2204     
- Misses       3465    5698     +2233     
- Partials      135     151       +16

Impacted Files	Coverage Δ
src/client/language/iterableTextRange.ts	12.5% <12.5%> (ø)
src/client/providers/docStringFoldingProvider.ts	7.35% <7.35%> (ø)
src/client/extension.ts	83.07% <83.07%> (-12.96%)	⬇️
...nittests/common/testVisitors/resultResetVisitor.ts	9.37% <0%> (-90.63%)	⬇️
...nt/unittests/common/services/testResultsService.ts	4.28% <0%> (-90%)	⬇️
...lient/unittests/unittest/services/parserService.ts	9.09% <0%> (-88.64%)	⬇️
...nt/unittests/unittest/services/discoveryService.ts	13.04% <0%> (-86.96%)	⬇️
src/client/common/terminal/service.ts	13.72% <0%> (-86.28%)	⬇️
src/client/unittests/pytest/runner.ts	15% <0%> (-85%)	⬇️
src/client/unittests/unittest/runner.ts	11.01% <0%> (-82.21%)	⬇️
... and 102 more

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