Annotation for code blocks that aren't expected to compile

[saecki]

Currently packages often use the typ language for readme examples that aren't valid typst code, or don't compile successfully, e.g.

```typ
#let my-func(
  a: int,
  b: str,
) = { ... }
```

This is a proposal to explicitly annotate these examples with some sort of tag, for example ignore, no-run, should-fail, or even more complex expressions such asexpect-failure=syntax, error=eval, to indicate they won't compile:

```typ ignore
#let my-func(
  a: int,
  b: str,
) = { ... }
```

This will even leave syntax highlighting intact:

#let my-func(
  a: int,
  b: str,
) = { ... }

The purpose of adding these tags, is to allow treating all other typ code blocks as tests that should compile.