SQL: make foreign tags for 'interenal' functions

[masatake]

CREATE FUNCTION isnlt(ean13, ean13)
        RETURNS boolean
        AS 'int8lt'
        LANGUAGE 'internal'
        IMMUTABLE STRICT
        PARALLEL SAFE;

This change extracts int8lt as a function in the C language.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.91%. Comparing base (fe86559) to head (af013a5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4240      +/-   ##
==========================================
+ Coverage   85.88%   85.91%   +0.02%     
==========================================
  Files         243      243              
  Lines       62764    62781      +17     
==========================================
+ Hits        53904    53937      +33     
+ Misses       8860     8844      -16     

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