Document the pathvar command about nushell.github.io HOT 2 OPEN

#188 adds an example for pathvar add in the table, but perhaps we should add a Path section to the Environment page in the book? I'm currently documenting nushell/nushell#3950 on that page and I would gladly add information about working with the path env var as well.

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In general, we could use some more information about manipulating the PATH in Nu.

We currently have pathvar, which makes it very easy to add and remove specific paths. However, it is also possible to change the path by setting a value to the PATH env var (or Path on Windows). I don't know if this is intended or not?

> pathvar add ~/.local/bin
# works!

> let-env PATH = (pathvar | prepend ("~/.local/bin" | path expand) | str collect ":")
# also works!

Both of the above do the same thing. The second one is closer to what we usually see in other shells (and can be done with string concatenation to make it resemble that pattern even more).

Setting the PATH all at once as a string has its use cases, like restoring an old PATH when you don't know the number of elements you would like to remove. This happens when you're working with some virtual environments that add more than one path element.

I would like to document this, if it's intended to work like this.

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