Allow specifying a file when running from stdin to use in place of `stdinfile.nim`
### Summary

Have a CLI switch like `--actualfile` which specifies the file that the input represents. Then all error messages and resolving config files will use that file path instead

### Description

For tooling based on `nim check` its easier to store the file in memory and pass it via stdin. This has a few limitations
- Error messages report as being from `stdinfile.nim` and so need to be converted before being shown to the user
- Config files that are `$file.nims` are not loaded
- `currentSourcePath()` returns `stdinfile.nim` so checking code that uses files relative to the source file always fails

The flag should make the compiler treat the input as coming from an actual file and use that when referencing to errors inside the input and resolving other files like configs

### Alternatives

The Nim compiler API could be used instead but this has the problems of
- Not being able to cancel checking as easily as just killing a process
- Tooling is then linked to the version of Nim that it was built with, user then has to recompile every Nim update

Also could use nimsuggest which supports a `dirtyfile` flag, but issues like #24413 make it a pain

### Examples

`echo "something()" | nim check --actualfile:foo.nim -`

```
/the/current/working/directory/foo.nim(1, 1) Error: undeclared identifier: 'something'
```

`echo "something()" | nim check --actualfile:/absolute.nim -`
```
/absolute.nim(1, 1) Error: undeclared identifier: 'something'
```



### Backwards Compatibility

Shouldn't cause any problems

### Links

_No response_