Normalised `open` doesn't work as expected on WSL2

Nobody has claimed this yet.

First of thanks for sharing your dotfiles, it's been a major source of inspiration to setup my WSL2 machine lately.

So, I tried using the normalised [`open`](https://github.com/mathiasbynens/dotfiles/blob/66ba9b3cc0ca1b29f04b8e39f84e5b034fdb24b6/.functions#L144) _(line #144 of the `.functions` file_) code snippet on a WSL2 Ubuntu distro & it won't work as expected. I suspect `grep` can't read the `/proc/version` file due to missing `sudo` privileges(?). I bet a better solution would be to `grep` the output of `uname -r` which doesn't require `sudo` privileges.

**A possible "_better_" solution could be**:

```bash
if [ ! $(uname -s) = 'Darwin' ]; then
if [ $(uname -r | grep -i 'microsoft') ]; then
# Ubuntu on Windows using the Linux subsystem
alias open='explorer.exe';
else
alias open='xdg-open';
fi
fi
```

EDIT: Provided a "_better_" solution & I don't mind opening a PR with the above changes 😄

Start in the normalized `open` snippet at `.functions` line 144 and compare its `/proc/version` detection with the proposed `uname -r` check. Test the behavior on a WSL2 Ubuntu distro, including the `explorer.exe` and `xdg-open` paths; done means `open` selects the expected command without requiring elevated privileges.

Written by the indexing model from the issue text.