🐛 Warn on dynamic function with surrounding text

[chrisjsewell]

Currently, for an item in a list field, with a dynamic function plus other text, the other text will be silently omitted, e.g.

.. spec:: TEST_1
    :id: TEST_1
    :tags: test_1a;test_1b;[[copy('title')]]omitted

A warning is now emitted about this

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.55%. Comparing base (4e10030) to head (12fb6c0).
Report is 123 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1426      +/-   ##
==========================================
+ Coverage   86.87%   88.55%   +1.67%     
==========================================
  Files          56       60       +4     
  Lines        6532     7236     +704     
==========================================
+ Hits         5675     6408     +733     
+ Misses        857      828      -29     

Flag	Coverage Δ
pytests	88.55% <100.00%> (+1.67%)	⬆️

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