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This removes the last three custom Babel plugins we had and replaces them with estree versions. Furthermore, it removes `@babel/generator`. For the plugins, we were only looking at ESM import/exports, but right now we’re delegating work to `periscopic` to look at which things are defined in the top-level scope. It’s a bit more complex, but this matches better with intentions, fixes some bugs, and prepares for a potential future where other ES constructs are allowed, so all in all should be a nice improvement. For serializing, we’re switching to `astring`, and handling JSX for now internally (could be externalized later). `astring` seems fast and is incredibly small, but is not very popular. We might perhaps see bugs is serialization in the future because of that, but all our tests seem fine, so I’m not too worried about that. Estree remains a somewhat fragmented ecosystem, such as that the tree walkers in `periscopic` and `astring` are different, so we might also consider writing our own serializer in the future. Or, when we implement Babel’s React JSX transform ourselves, could switch to another generator, or at least drop the JSX serialization code here. Because of these changes, we can drop `@babel/core` and `@babel/generator` from `@mdx-js/mdx`, which drops the bundle size of from 349kb to 111kb. That’s 68%. Pretty nice. This should improve downloading and parsing time of bundles significantly. Of course, we currently still have JSX in the output, so folks will have to resort to Babel (or `buble-jsx-only`) in another step. For performance, v2 (micromark) was already an improvement over v1. On 1000 simple files totalling about 1mb of MDX: * v1: 3739ms * v2: 2734ms (26% faster) * v2 (w/o babel): 1392ms (63% faster). Of course, this all really depends on what type of stuff is in your MDX. But it looks pretty sweet! ✨ Related to GH-1046. Related to GH-1152. Related to GH-1338. Closes GH-704. Closes GH-1384.
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