[PoC] V2 API changes using cargo feature flag

[kdy1]

Description:

We can use multi-branch but I think it will really hurt rust-side users.
With multi-branch approach, we have to prevent some patches from being published to crates.io, but I think it's not acceptable for some rust-side users.

So I'm suggesting another approach. We can use a (potentially private) feature flag named swc_v2 in js-surfacing crates like swc or swc_node_bundler to 'hide' some changes to node api.

This is clearly not additive, and that's why I think it should be a private feature.
As users are not expected to depend on swc_v2, enabling it from outside of SWC may break the whole dependency tree, but it's fine.

For PoC, I fixed #5527 only for v2.

I want to hear more about opinions about this approach

Some constraints

• rust users should not depends on swc_v2. This is not considered as a public api.

• All code should be accessible witthout swc_v2.

But altering behavior is fine.
Let's say, we want to fix #5535 in v2 and change the order of plugins to after resolver but before decorator or typescript::strip. In this case we can use #[cfg(feature = "swc_v2")] and #[cfg(not(feature = "swc_v2")] respectively.

Related issue (if exists):

[kwonoj]

At this point, I think we'll probably need to try couple of different approaches to find what's best to suit our needs. Branch, features both have pros and cons and probably one approach will not fulfill most of our needs. The way I'm thinking is probably we'll use feature for the additive (i.e new things on top of sharable), and branch for the change / remove. Some of questions regarding feature-based major is some cases like if involved change requires to keep lot of repeated code with small amount of changes, or how to control deeply transitive dep's (i.e core -> A -> B -> C -> D and we need change D, whereas A / B/ C potentially have D as well), or something else.

Double clarify this is not an objection, just saying we probably need to collect real world challenges before make a firm decision on one end.