[encoding/json] Allow Unmarshalling to rune

Fixes https://github.com/odin-lang/Odin/issues/6061

core/encoding/json/types.odin

@@ -76,6 +76,7 @@ Error :: enum {
 	Invalid_Number,
 	String_Not_Terminated,
 	Invalid_String,
+	Invalid_Rune,
 
 
 	// Parsing Errors

core/encoding/json/unmarshal.odin

@@ -225,6 +225,15 @@ unmarshal_string_token :: proc(p: ^Parser, val: any, token: Token, ti: ^reflect.
 		}
 		ok = true
 		return
+	case rune:
+		for rne, i in str {
+			if i > 0 {
+				dst = {}
+				return false, .Invalid_Rune
+			}
+			dst = rne
+		}
+		return true, nil
 	}
 	
 	#partial switch variant in ti.variant {