[JS API] organize types into enums

[chharvey]

Alternative to #9085, which renamed binaryen.none to binaryen.void
and added a new binaryen.none referring to the heap type.
There were concerns that binaryen.void mapping to
the C API BinaryenTypeNone() might be confusing.

This PR moves all types from the top-level binaryen.* namespace
into respective enums called Type, HeapType, and PackedType.

This change plays nicely with AssemblyScript’s established
TypeScript typings
because now types are scoped in an enum space.
This is not a breaking change for the TS typings file —
e.g., a function that accepts a Type (previously just number)
now accepts a member of the new Type enum.

This change also follows well-established patterns of existing enums
such as ExpressionIds and Features.

binaryen.Type.none now uses BinaryenTypeNone() and
binaryen.HeapType.none uses BinaryenHeapTypeNone(),
so there’s no conflict across enums.

This is a breaking change for JS/TS users:
they will now have to reference types from the enum space
instead of at the top level.
E.g. binaryen.i32 becomes binaryen.Type.i32,
binaryen.any becomes binaryen.HeapType.any,
binaryen.i8 becomes binaryen.PackedType.i8, etc.

[kripken]

lgtm, but let's see what others think about the API and TypeScript stuff here. cc @brendandahl

[tlively]

This seems like a good direction to me.