Slim System in user nim.cfg breaks nimble
NOTE: this clobbers your `nim.cfg` if that matters to you and may need leading directory creation..
```sh
echo define:nimPreviewSlimSystem > $HOME/.config/nim/nim.cfg
nimble install nimble
```
yields
```
Downloading https://github.com/nim-lang/nimble using git
       Tip: 4 messages have been suppressed, use --verbose to show them.
packageparser.nim(393)   readPackageInfo

    Error:  Could not read package info file in /tmp/nimble_17752/githubcom_nimlangnimble/nimble.nimble;
        ...   Reading as ini file failed with:
        ...     Invalid section: .
        ...   Evaluating as NimScript file failed with:
        ...     /tmp/nimblecache-1979690426/nimscriptapi_1508423453.nim(168, 7)
Error: undeclared identifier: 'writeFile'
        ... candidates (edit distance, scope distance); see '--spellSuggest':
        ...  (4, 4): 'outFile'
        ...  (4, 4): 'scriptFile'
        ...  (4, 6): 'moveFile'
        ...  (4, 6): 'rmFile'
        ...  (4, 6): 'sameFile'
        ...  (4, 6): 'splitFile'
        ... printPkgInfo() failed.
```
then if you `rm $HOME/.config/nim/nim.cfg` it works fine.

Contextual version information:
```$ nim -v
Nim Compiler Version 1.9.1 [Linux: amd64]
Compiled at 2022-12-24
Copyright (c) 2006-2022 by Andreas Rumpf

active boot switches: -d:release -d:danger -d:nimUseLinenoise -d:nimHasLibFFI
```
but this has been a problem for me ever since I started testing `nimPreviewSlimSystem`.

Could be as easy as adding `std/syncio` .. somewhere.