fix: export is_subfield_normal and principal_subfields

[thofma]

Closes #1656

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.81%. Comparing base (2af99ca) to head (44fcdcf).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1657      +/-   ##
==========================================
- Coverage   75.85%   75.81%   -0.04%     
==========================================
  Files         361      361              
  Lines      113719   113718       -1     
==========================================
- Hits        86262    86216      -46     
- Misses      27457    27502      +45     

Files with missing lines	Coverage Δ
src/NumField/NfAbs/NfAbs.jl	68.90% <ø> (ø)
src/NumField/SimpleNumField/Subfields.jl	92.40% <ø> (ø)

... and 26 files with indirect coverage changes