Fix JSClosure leak

[kateinoigakukun]

letc1=JSClosure{ _ in.undefined }
consume c1

did not release the JSClosure itself and it leaked the underlying
JavaScript closure too because JSClosure -> JS closure thunk ->
Closure registry entry -> JSClosure reference cycle was not broken
when using FinalizationRegistry. (Without FR, it was broken by manual
release call.)

Note that weakening the reference does not violates the contract that
function reference should be unique because holding a weak reference does
deinit but not deallocate the object, so ObjectIdentifier is not reused
until the weak reference in the registry is removed.

[github-actions]

Time Change: +325ms (3%)

Total Time: 9,697ms

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import	23ms	+2ms (6%)	🔍
Serialization/JavaScript function call through Wasm import with int	16ms	+2ms (9%)	🔍
Serialization/JavaScript function call from Swift	103ms	+7ms (7%)	🔍
Serialization/JavaScript Number to Swift Int	331ms	+32ms (9%)	🔍

View Unchanged

Test name	Duration	Change
Serialization/Swift Int to JavaScript with assignment	334ms	+17ms (4%)
Serialization/Swift Int to JavaScript with call	993ms	+32ms (3%)
Serialization/Swift String to JavaScript with assignment	391ms	+8ms (2%)
Serialization/Swift String to JavaScript with call	1,048ms	+30ms (2%)
Serialization/JavaScript String to Swift String	3,821ms	+158ms (4%)
Object heap/Increment and decrement RC	2,625ms	+37ms (1%)

View Baselines

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

[kateinoigakukun]

Tested with a random large-scale application and no major regression found