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).

build.zig.zon

@@ -13,8 +13,8 @@
             .hash = "tigerbeetle_io-0.0.0-ViLgxpyRBAB5BMfIcj3KMXfbJzwARs9uSl8aRy2OXULd",
         },
         .v8 = .{
-            .url = "https://github.com/lightpanda-io/zig-v8-fork/archive/363e2899e6d782ad999edbfae048228871230467.tar.gz",
-            .hash = "v8-0.0.0-xddH6wHzIAARDy1uFvPqqBpTXzhlnEGDTuX9IAUQz3oU",
+            .url = "https://github.com/lightpanda-io/zig-v8-fork/archive/f0c7eaaffe39f2f1a224fbe97e550daca0ca1801.tar.gz",
+            .hash = "v8-0.0.0-xddH62T4IADchAHFgo4nx79w1VedNDhIVErtSNgup-Tk",
         },
         //.v8 = .{ .path = "../zig-v8-fork" },
         //.tigerbeetle_io = .{ .path = "../tigerbeetle-io" },