Add news for LS fixes

[MikhailArkhipov]

Add flag for upcoming LS startup time improvement.

• Title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Unit tests & code coverage are 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)
• Dependencies are pinned (e.g. "1.2.3", not "^1.2.3")

[codecov]

Codecov Report

Merging #2372 into master will decrease coverage by 0.39%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #2372     +/-   ##
========================================
- Coverage      76%   75.6%   -0.4%     
========================================
  Files         309     309             
  Lines       14331   14331             
  Branches     2537    2537             
========================================
- Hits        10892   10835     -57     
- Misses       3429    3486     +57     
  Partials       10      10

Flag	Coverage Δ
#MacOS	?
#Windows	?

Impacted Files	Coverage Δ
src/client/activation/languageServer.ts	26.08% <ø> (ø)	⬆️
src/client/common/platform/registry.ts	40.42% <0%> (-51.07%)	⬇️
src/client/common/platform/pathUtils.ts	66.66% <0%> (-33.34%)	⬇️
src/client/common/utils.ts	57.89% <0%> (-18.43%)	⬇️
src/client/common/platform/serviceRegistry.ts	88.88% <0%> (-11.12%)	⬇️
src/client/common/platform/platformService.ts	84.21% <0%> (-5.27%)	⬇️
src/client/common/process/pythonProcess.ts	88.63% <0%> (-4.55%)	⬇️
src/client/refactor/proxy.ts	81.05% <0%> (-3.16%)	⬇️
src/client/unittests/unittest/socketServer.ts	79.45% <0%> (-2.74%)	⬇️
...interpreter/locators/services/KnownPathsService.ts	92.68% <0%> (-2.44%)	⬇️
... and 6 more

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[d3r3kk]

Looks fine, but do we want to set up a setting that the user can manipulate the new asyncStartup: true parameter?

[d3r3kk]

Do we want this hard-coded in this way? Should there be a user setting in VScode for this?

[patrys]

Should this also mention #2017?

[MikhailArkhipov]

@patrys - thanks, added.