feat(babel): API for custom template tags

[Anber]

Motivation

atomic highlighted the complexity of tags' processing logic. That part hasn't been changed since the beginning of Linaria. So, the time for changes has finally come.

Summary

This PR introduces a new way of creating tags. Each tag (both css and styled, as well as their atomic versions) now has a processor that implements the abstract class BaseProcessor. Those processors incapsulate all tag logic and provide the API for the generic template processor inside babel-preset.

Test plan

A lot of new tests were added.

[Anber]

@juanferreras @jpnelson
Hi guys!

I did some bad things here:

1. atomize option was removed, since it isn't needed anymore. You don't need to configure anything, just import tags form @linaria/atomic and it should just work. But it's actually good news :)
2. The bad news, is that I had to drop libResolver as well since it became unclear how to resolve tag processors in case of re-exports. What are your use cases for libResolver? Maybe we can find a better alternative.

And the main news is that it is become possible to make custom tags without touching Linaria's insides. The API is unstable yet and TBD, but a start has been made.

[juanferreras]

@juanferreras @jpnelson Hi guys!

I did some bad things here:

1. atomize option was removed, since it isn't needed anymore. You don't need to configure anything, just import tags form @linaria/atomic and it should just work. But it's actually good news :)
2. The bad news, is that I had to drop libResolver as well since it became unclear how to resolve tag processors in case of re-exports. What are your use cases for libResolver? Maybe we can find a better alternative.

And the main news is that it is become possible to make custom tags without touching Linaria's insides. The API is unstable yet and TBD, but a start has been made.

Hi @Anber ! These last refactors you've been working on seem like fantastic iterations!

I don't have anything against either of these points, I can quote @jpnelson on why I think he went for options.atomize

[...] Invert the dependency so that @linaria/atomic has the atomize function, and the postcss dependency. The atomize function can be a part of the linaria config used by the babel preset, and if you want to use atomic styles, pass in atomize: require('@linaria/atomic').atomize in the config

See #867 (comment) for your original comment too. If you've managed to solve this, then this is indeed actually great news to simplify the usage!

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I haven't used libResolver on my end – this is the original issue from @jpnelson #851

[Anber]

If you've managed to solve this, then this is indeed actually great news to simplify the usage!

Yep, that was the aim of this refactoring.

I haven't used libResolver on my end – this is the original issue from @jpnelson #851

Maybe it's possible to re-import from the local project by adding something like this to package.json:

  "linaria": {
    "tags": {
      "css": "@linaria/core/processors/css.js",
      "styled": "@linaria/react/processors/styled.js"
    }
  },