Capture its unique properties:
1- instances are falsy, and
2- instance can be called as a function
The behavior is used for browser detection (i.e. duckduckgo treats us as a
legacy browser because we document.all != false)
There's ambiguity in mapping due to the flexible nature of JavaScript. Hopefully
most types are unambiguous, like a string or am *parser.Node.
We need to "probe" each field to see if it's a possible candidate for the JS
value. On a perfect match, we stop probing and set the appropriate union field.
There are 2 levels of possible matches: candidate and coerce. A "candidate"
match has higher precedence. This is necessary because, in JavaScript, a lot
of things can be coerced to a lot of other, seemingly wrong, things.
For example, say we have this union:
a: i32,
b: bool,
Field `a` is a perfect match for the value 123. And field b is a coerce match
(because, yes, 123 can be coerced to a boolean). So we map it to `a`.
Field `a` is a candidate match for the value 34.2, because float -> int are both
"Numbers" in JavaScript. And field b is a coerce match. So we map it to `a`.
Both field `a` and field `b` are coerce matches for "hello". So we map it to `a`
because it's declared first (this relies on how Zig currently works, but I don't
think the ordering of type declarations is guaranteed, so that's an issue).
Remove Env from caller, and don't store Env in isolate. We don't need it, we
can execute all runtime code from the Executor (which we store a pointer to
in the v8.Context)
