Ensure Outline view does not overload the symbol provider

[DonJayamanne]

Fixes #1856

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)

[codecov]

Codecov Report

Merging #1858 into master will increase coverage by 0.14%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master    #1858      +/-   ##
==========================================
+ Coverage   74.29%   74.44%   +0.14%     
==========================================
  Files         282      282              
  Lines       13189    13209      +20     
  Branches     2371     2374       +3     
==========================================
+ Hits         9799     9833      +34     
+ Misses       3258     3244      -14     
  Partials      132      132

Impacted Files	Coverage Δ
src/client/providers/symbolProvider.ts	82% <89.47%> (+58.66%)	⬆️

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