ENH: add outline_count property

[mtd91429]

Enables retrieval of "/Count" attribute of outline item in PdfReader by implementing property outline_count. Addresses #1122

[codecov]

Codecov Report

Merging #1129 (e2b46d7) into main (33634d4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1129   +/-   ##
=======================================
  Coverage   92.02%   92.03%           
=======================================
  Files          24       24           
  Lines        4666     4671    +5     
  Branches      964      965    +1     
=======================================
+ Hits         4294     4299    +5     
  Misses        227      227           
  Partials      145      145           

Impacted Files	Coverage Δ
PyPDF2/_reader.py	91.48% <100.00%> (+0.02%)	⬆️
PyPDF2/generic.py	91.72% <100.00%> (+0.02%)	⬆️

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[MartinThoma]

Thank you for the nice PR and the detailed issue description 🤗

I absolutely love such PRs 😍

(1) Purpose: Due to the issue, I know the desired outcome / why the PR was made
(2) Focus: The PR is only about this part, so I only need to decide if that goal was reached and if the code is in a reasonable state
(3) Tests: The test means I don't need to add other work it it - the test already shows that it works as desired

Very well done 👍