Json: schema validation and assignment to struct

[staycoolcall911]

The following sections should be added to the language reference:

1.1.4.5 Schema validation

All fromJson() methods will validate that the runtime type is compatible with the target type in
order to ensure type safety (at a runtime cost):

```TS
str.fromJson(jsonNumber);      // RUNTIME ERROR: unable to parse number `123` as a string.
num.fromJson(Json "\"hello\""); // RUNTIME ERROR: unable to parse string "hello" as a number

For each fromJson(), there is a tryFromJson() method which returns an optional T? which
indicates if parsing was successful or not:

let s = str.tryFromJson(myJson) ?? "invalid string";

1.1.4.7 Assignment to user-defined structs

All structs also have a fromJson() method that can be used to parse Json into a
struct:

struct Contact {
  first: str;
  last: str;
  phone: str?;
}

let j = Json { first: "Wing", last: "Lyly" };
let myContact = Contact.fromJson(j);
assert(myContact.first == "Wing");

When a Json is parsed into a struct, the schema will be validated to ensure the result is
type-safe:

let p = Json { first: "Wing", phone: 1234 };
Contact.fromJson(p);
// RUNTIME ERROR: unable to parse Contact:
// - field "last" is required and missing
// - field "phone" is expected to be a string, got number.

[hasanaburayyan]

@staycoolcall911 this needs bumped up to P1 before Beta right? Well at least the struct.fromJson() bit

[staycoolcall911]

Yes, definitely, this came up much during dogfooding. Bumping to p1

[hasanaburayyan]

Splitting out some features into different issues that wont be covered by initial work in: #3648

additional issues:

• Json: schema validation disable partial parsing #3789
• Json: support unsafe schema validation #3788
• Json: schema validation expose schema() object #3790
• Json: schema validation support fromJson for imported structs #3792

[monadabot]

Congrats! 🚀 This was released in Wing 0.25.1.