Json: stricter number parsing

[D-Berg]

This PR makes zigs json.parseFromSlice return error UnexpectedToken when trying to parse a 'numbered string' when expecting an integer. For example the below test succeeds and parses "2134071" to an i32, while I believe that it should fail since the type of "number" is a json string.

test "parse json" {
    const Request = struct {
        method: []const u8,
        number: i32,
    };

    const allocator = std.testing.allocator;
    const req = 
        \\{"method":"isPrime","number":"2134071"}
    ;

    const parsed = try std.json.parseFromSlice(Request, allocator, req, .{});
    defer parsed.deinit();
}

Both Rust's Serde and Go's json are stricter and fail when trying to parse json 'numbered strings' to numbers and this would make zigs json lib conform to the expected behaviour of those other languages libs and make it up to the user to then parse the string to a number.