Make all modules in function crate public, add Default impl's for all Func's

[Omega359]

Is your feature request related to a problem or challenge?

Some of the modules in the functions crate are not public - primarily in the string and unicode modules - and do not follow the pattern set in #9903. As well, many of the Func's do not have a Default impl which should be added as well

Describe the solution you'd like

All modules that should be made public are public in the functions crate. All *Func's structs that are the basis for a ScalarUDFImpl should have Default impl's.

Describe alternatives you've considered

No response

Additional context

No response

[Omega359]

take