support function unnesting for json_as_text inner calls

[davidhewitt]

Noticed this while investigating #53

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.78%. Comparing base (6fae049) to head (0cb8be7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   84.69%   84.78%   +0.09%     
==========================================
  Files          15       15              
  Lines         980      986       +6     
  Branches      980      986       +6     
==========================================
+ Hits          830      836       +6     
  Misses        103      103              
  Partials       47       47              

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

[davidhewitt]

I think the justification here is that at the moment for x->>a->>b, we only require x and x->>a to be text, which means that this works fine. In a world with a custom JSON type, we might require a JSON type as input, so x-->a returning a string (i.e. stripping the JSON-ness) would then cause the subsequent ->>b lookup to be invalid.