Allow facet-json to build with no-default-features

[itsjunetime]

facet-json doesn't currently build when default-features = false because that disables the std features of all the underlying crates, and no-std/no-alloc doesn't necessarily compile in all of the facet crates atm.

It's also just not reasonable to expect a general-purpose json serializer/deserializer to work out-of-the-box without std at the moment, so I think this is the correct solution to this problem.

[github-actions]

📦 limpid report

Number of crates: 51 (➖ no change)
8350 symbols (➖ no change), totaling 885.5 KB (➖ no change)

Symbol	Baseline Size	Current Size	Change
facet_core::types::def::list::ListDef::t found in facet_core	3 B	—	🗑️ REMOVED
facet_core::types::def::set::SetDef::t found in facet_core	—	3 B	🆕 NEW

All significant changes are listed above.

Number of LLVM lines: 351,318 (➖ no change)
Wall duration: 6.12 s (📉 -0.21 s)