Stop use of global variable as a object cache

[kateinoigakukun]

Fix crash on workers due to use of JSObject referencing main thread objects

[github-actions]

Time Change: -149ms (1%)

Total Time: 9,611ms

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import	17ms	-8ms (45%)	🎉
Serialization/JavaScript function call through Wasm import with int	14ms	+1ms	🔍
Serialization/JavaScript Number to Swift Int	297ms	-15ms (5%)	✅
Serialization/Swift String to JavaScript with assignment	384ms	-41ms (10%)	👏

View Unchanged

Test name	Duration	Change
Serialization/JavaScript function call from Swift	109ms	-1ms (1%)
Serialization/Swift Int to JavaScript with assignment	333ms	-10ms (3%)
Serialization/Swift Int to JavaScript with call	1,033ms	-40ms (3%)
Serialization/Swift String to JavaScript with call	1,082ms	-29ms (2%)
Serialization/JavaScript String to Swift String	3,685ms	+1ms (0%)
Object heap/Increment and decrement RC	2,642ms	-7ms (0%)

View Baselines

Test name	Duration
Serialization/Call JavaScript function directly	3ms
Serialization/Assign JavaScript number directly	2ms
Serialization/Call with JavaScript number directly	3ms
Serialization/Write JavaScript string directly	3ms
Serialization/Call with JavaScript string directly	2ms