Annotation for code blocks that aren't expected to compile

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.

Begin by locating the code that identifies and checks Typst code blocks; the issue names no file or test to start from. Define the accepted annotation semantics, then verify that tagged blocks are exempt while untagged typ blocks remain compile checks.