✨ Add c.this_doc() check for needextend filters

[chrisjsewell]

This allows for filtering of needs only in the same document as the needextend itself, e.g.

.. needextend:: c.this_doc() and status is None
   :status: open

Putting this under the c (for check) namespace allows for the possibility to add future special checks, without polluting the global namespace.

cc @arwedus

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.24%. Comparing base (4e10030) to head (5727e65).
Report is 109 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/filter_common.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1393      +/-   ##
==========================================
+ Coverage   86.87%   88.24%   +1.36%     
==========================================
  Files          56       60       +4     
  Lines        6532     7155     +623     
==========================================
+ Hits         5675     6314     +639     
+ Misses        857      841      -16     

Flag	Coverage Δ
pytests	88.24% <90.00%> (+1.36%)	⬆️

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[ubmarco]

Test case would be nice to have.

[chrisjsewell]

Test case would be nice to have.

done

[ubmarco]

Good change