Initial support for es module exports

[belgattitude]

Do not merge yet. I'd like to add more ci tests. Dual packaging is hard when going into details.

Rework publishing iteration 1.

• Add es module exports.
• Keep dual packaging (and packaging hazards that comes along)

Why ?

It paves the way to:

• Avoid duplicates instances of react-i18next in some situations (at least in the long run and esm only)
• Prepare a shift to esm only. Warning: recent nextjs will work, not older ones. Jest is almost there, but there's probably a lot of old installs. So I'll try to keep a conservative approach
• Explicitly declare subpaths

Version

Minor version. Should not break anything.

Risks

Dual packaging is always tricky. There's risks to add the exports field. Out of my mind (not exhaustive):

• Typescript (< 4.9) still have issues when dealing with types and subpaths exports (like serverSideProps). Fixed by typeConfig
• Older webpack consider the non-standard 'import' field (rather than modules), so I kept it there with comment

Next iterations

I'll detail soon. But I see a lot of potential for either size-reduction, either normalization. Other packages i18next, react-i18next will have to be updated too to bring full potential.

Size history

Before:

After:

No size regression

[belgattitude]

I prefer to not expose yet the browsersList into package.json. In next pr iteration I'll probably move this.

[belgattitude]

Credits to @andrewbranch, see https://github.com/andrewbranch/example-subpath-exports-ts-compat#strategy-support-matrix. Huge thanks

[belgattitude]

@adrai I've tested on few IDE and seems to work fine (at least with a recent typescript version)

It's a first step, I tried to not optimize the bundle yet and keep what did exists. On the user side it does not bring improvements (yet) but there's a slight possibility of issues.

2 options:

• we merge it (so it will be included in v13) and I'll provide size-reduction on a 13.1. That way not everything is sent at the same time.
• we keep here for a while.

[belgattitude]

FYI: In a later step, I'd like to introduce rollup 3, completely rework/refresh the babel config, get real es modules (the current esm build is not)... Let me know the approach and if it's fine for you

[adrai]

I would wait for the feedback regarding the peer dep change first...
and then since it's a major we can release all together with v13...

[belgattitude]

Redone in #1998