Make JSValue's subscript setter nonmutating

[kateinoigakukun]

Close #132 based on #270

[github-actions]

Time Change: +263ms (2%)

Total Time: 9,936ms

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import with int	22ms	+4ms (18%)	⚠️
Serialization/Swift Int to JavaScript with assignment	332ms	-22ms (6%)	✅
Serialization/JavaScript Number to Swift Int	263ms	-13ms (5%)	✅
Serialization/Swift String to JavaScript with assignment	408ms	-24ms (5%)	✅
Serialization/JavaScript String to Swift String	3,826ms	+266ms (6%)	🔍

View Unchanged

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import	23ms	-1ms
Serialization/JavaScript function call from Swift	129ms	-5ms (4%)
Serialization/Swift Int to JavaScript with call	977ms	-29ms (2%)
Serialization/Swift String to JavaScript with call	1,062ms	-6ms (0%)
Object heap/Increment and decrement RC	2,863ms	+97ms (3%)

View Baselines

Test name	Duration
Serialization/Call JavaScript function directly	8ms
Serialization/Assign JavaScript number directly	8ms
Serialization/Call with JavaScript number directly	5ms
Serialization/Write JavaScript string directly	6ms
Serialization/Call with JavaScript string directly	5ms