Support `context`

Hello!

I'm rather new to Typst, so please forgive me if what I'm saying doesn't make sense or is otherwise insane!

I was having issues where Typst would compile a document fine, but Pandoc would not. It appeared to be an issue with library code in CeTZ

This appears to reproduce the issue:

```typst
#let fun() = context {
_ = "hi"
}
```

However, both of these are fine:

```typst
#let fun() = {
_ = "hi"
}
```

```typst
#let fun() = context {
x = "hi"
}
```

Additionally, this compiles fine, leading me to believe that `_` might be misinterpreted as the beginning of an emphasis marker:

```typst
#let fun() = context {
__ = "hi"
}
```

No repository file or test is named. Start by running the three Typst snippets from the issue through the parser/evaluator and compare the failing `context` case with the working block cases; done means the failing example is accepted without breaking the existing cases.

Written by the indexing model from the issue text.