Ensure search for jupyter commands are cancelled early

[DonJayamanne]

For #7262

Basically the cancellation Token isn't checked everytime.
So even if the user were to click cancel in #7262, we'd still be doing a lot of processing.

Will implement solution for #7262 in another PR (wanted to keep this simple).

[codecov-io]

Codecov Report

Merging #7714 into master will decrease coverage by 0.03%.
The diff coverage is 42.42%.

@@            Coverage Diff             @@
##           master    #7714      +/-   ##
==========================================
- Coverage   58.72%   58.68%   -0.04%     
==========================================
  Files         494      494              
  Lines       22027    22053      +26     
  Branches     3541     3550       +9     
==========================================
+ Hits        12935    12942       +7     
- Misses       8281     8294      +13     
- Partials      811      817       +6

Impacted Files	Coverage Δ
...client/datascience/jupyter/jupyterCommandFinder.ts	69.53% <38.88%> (-4.73%)	⬇️
src/client/common/cancellation.ts	30.55% <46.66%> (-1.45%)	⬇️

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[DonJayamanne]

Close and open due to busted CI

[rchiodo]

[IanMatthewHuff]