feat: add printer.preprocess

[ikatyang]

Sometimes we need to transform the ast to make it easier to print, but it's currently done in the parser (markdown and yaml), which leads the output ast harder to use for external users (custom parser), adding printer.preprocess can solve this issue.

And also this way we could move the trailing newline for json from ast-to-doc.js to language-js.

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• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory)
• I’ve read the contributing guidelines.

[suchipi]

When a user uses a custom parser function, do we call preprocess before or after their function?

[ikatyang]

text -> parser.preprocess -> parser (custom parser) -> printer.preprocess -> printer