Support for Typescript

[TianyiShi2001]

Yet another engine.

[yihui]

I'll be happy to accept this PR. My only problem is the repetition of code from other engine functions, which indicates that we'll need some refactoring at some point (to clarify, this is not your responsibility).

BTW, please discard changes in tweak_bib.csv. Thanks!

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[yihui]

As I just mentioned in #1829 (comment) about the new exec engine, now you can use

```{exec, command='ts-node'}
```

to run TypeScript code. Thanks!