♻️ Make needextend argument declarative

[chrisjsewell]

The argument for needextend can refer to either a single need ID or filter function.
Currently, the format cannot be known until all needs have been processed, and it is resolved during post-processing.
This is problematic for (a) user readability, (b) improving processing performance and issue feedback

This PR slightly modifies the argument processing to allow for two "explicit" formats:

• <ID>, if the argument is enclosed in <> it is always processed as a single ID
• "filter string", if the argument is enclosed in "" it is always processed as a filter string

If the string is not enclosed, then the logic is relatively similar to the existing:

• ID, if the argument is a single word (no spaces) then it is processed as a single ID
• otherwise, it is processed as a filter string

This change should not break most uses of needextend, but there are some corner-cases that would need to be adapted, e.g. a single is_external would now be processed as an ID, unless it was enclosed or expanded:

.. needextend:: "is_external"
.. needextend:: is_external is True

[codecov]

Codecov Report

Attention: Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.24%. Comparing base (4e10030) to head (2902559).
Report is 106 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/directives/needextend.py	93.75%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1391      +/-   ##
==========================================
+ Coverage   86.87%   88.24%   +1.36%     
==========================================
  Files          56       60       +4     
  Lines        6532     7145     +613     
==========================================
+ Hits         5675     6305     +630     
+ Misses        857      840      -17     

Flag	Coverage Δ
pytests	88.24% <94.59%> (+1.36%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ubmarco]

Just 2 minor remarks