Fix: std.json: Writing inf values returns InvalidJSON error

[bradcypert]

I believe this should fix #23253. First time contributor here, so please feel free to be as thorough as you'd like with feedback.

Abstract: Writing an infinite float in JSON produces the value inf which is not valid JSON. Since we can not convert an infinite value to JSON, introduce a new error type named InvalidJson and return that when attempting to write a value that can not be converted to valid JSON.

[bradcypert]

It looks like one of the errors in CI is related to this: #22107 . Anyone with more experience working on the std lib have any thoughts? I can cast the comptime floats to regular floats before the checks, but perhaps isInf is intended to also support comptime floats? I suppose I could also split the float and comptime float checks into two separate blocks in the switch, too.

[richard-powers]

#22107 (comment)
Seems it intentionally doesn't support comptime floats

[linusg]

• You'll have to update the error sets of all stringify functions, not just stringifyAlloc
• The doc comment bulletpoint Zig floats -> JSON number or string. should be updated to mention this behavior
• error.InvalidJson does not accurately describe the failure - this is serialization to JSON and the input value is invalid, not the other way around