Remove constexpr requirement for glz::schema fields & cleaned JSON schema code

[stephenberry]

No description provided.

[stephenberry]

@jbbjarnason, since you helped so much with this, I wanted to let you know that I've dropped the constexpr construction requirement on glz::schema, this means that we can now use std::vector rather than std::span and it makes including examples and other unknown array length fields easier.

[stephenberry]

Hmm, I'm not sure why GCC is failing this when clang works.
Edit: It turns out GCC doesn't properly instantiate a global inline const value if it is not constexpr. The solution is to make a static value at the call site.