Support union parameters

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).

Dockerfile

@@ -5,7 +5,7 @@ ARG ZIG=0.14.0
 ARG ZIG_MINISIG=RWSGOq2NVecA2UPNdBUZykf1CCb147pkmdtYxgb3Ti+JO/wCYvhbAb/U
 ARG ARCH=x86_64
 ARG V8=11.1.134
-ARG ZIG_V8=v0.1.19
+ARG ZIG_V8=v0.1.20
 
 RUN apt-get update -yq && \
     apt-get install -yq xz-utils \