util: Support jsonencode/jsondecode as regular functions

[apjanke]

What do you think about supporting jsonencode/jsondecode when implemented as regular functions, in addition to just when they're built-ins? This would allow the jsonencode/jsondecode from jsonstuff (https://github.com/apjanke/octave-jsonstuff) or any other Matlab-compatible implementation to be used, in addition to JSONio. This would be nice for me because I'd rather use jsonstuff than JSONio. And there's a chance that jsonencode/jsondecode will eventually get merged into Octave core from one of these libraries, but might be regular functions instead of builtins there, so this would be forward-looking for future Octave versions. (Also, Matlab could always change the jsonencode/jsondecode implementation to use regular M-file wrapper functions, which would break the current detection.)

[gllmflndn]

Yes. JSONio is only here temporarily until we can assume Matlab is recent enough to have jsonencode/jsondecode and an agreement is reached in Octave:
https://savannah.gnu.org/bugs/?53100
Shouldn't the test with exist also look for an M-file (ie not only 3)? It would also perhaps be better if the checks were only done once with the outcome stored in a persistent variable (as a function handle).
Having jsonstuff pushed into Octave before the 6.1 release would be great. If there isn't a quick agreement of which C++ library to rely on, jsonencode could still be added as I guess everyone implemented it in pure M-code.

[apjanke]

You're right; it should be looking for both 3 and 5. I have amended this PR to do so, and to store the outcome in a persistent variable. (Though maybe you effectively wanted the entire if/elseif/.../end chain logic and all those tests stored in a persistent.)