On Windows, we used the `is-exeuctable` crate but on Unix, we
duplicated the check that it did, with one difference: We also
looked at whether or not it was a symlink.
The `is-executable` crate uses `std::fs::metadata` which follows
symlinks, so this scenario should never occur here, as it will
return the metadata for the target file.
Using the `is-executable` crate on both Unix and Windows lets us
make it non-optional. This lets us remove the `executable-support`
feature. (It is worth noting that this code didn't compile on
Windows when the `executable-support` feature was not specified.)
Right now, there is an alternate code path for `target_arch` being
`wasm32`. This isn't exactly correct as it should probably handle
something different for when the `target_os` is `wasi`.
In Nu we have variables (E.g. $var-name) and these contain `Value` types.
This means we can bind to variables any structured data and column path syntax
(E.g. `$variable.path.to`) allows flexibility for "querying" said structures.
Here we offer completions for these. For example, in a Nushell session the
variable `$nu` contains environment values among other things. If we wanted to
see in the screen some environment variable (say the var `SHELL`) we do:
```
> echo $nu.env.SHELL
```
with completions we can now do: `echo $nu.env.S[\TAB]` and we get suggestions
that start at the column path `$nu.env` with vars starting with the letter `S`
in this case `SHELL` appears in the suggestions.
