refactor(core): store v8::Global<v8::Context> in an Rc (#18749)

Alternative to https://github.com/denoland/deno/pull/18726.

This was suggested by @piscisaureus. It's a bit ugly, but it does the
work and makes cloning `JsRealm` very cheap, while not requiring 
invasive changes.

Also managed to remove some vector and `v8::Global` clones which yields
about 5% improvement in the "async_ops_deferred.js" benchmark.

This PR:
```
time 1689 ms rate 592066
time 1722 ms rate 580720
time 1629 ms rate 613873
time 1578 ms rate 633713
time 1585 ms rate 630914
time 1574 ms rate 635324
```

`main` branch:
```
time 1687 ms rate 592768
time 1676 ms rate 596658
time 1651 ms rate 605693
time 1652 ms rate 605326
time 1638 ms rate 610500
```

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