Add PowerShell snippets for postinstall

[tats-u]

#66

This PR allows Windows users to activate poery command without forcing them to google how to do it.

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Name	Status	Preview	Updated
install-python-poetry-org	✅ Ready (Inspect)	Visit Preview	Jul 17, 2022 at 9:54AM (UTC)

[tats-u]

Poetry (1.1.14) is installed now. Great!
To get started you need Poetry's bin directory (C:\Users\runneradmin\AppData\Roaming\Python\Scripts) in your `PATH`
environment variable.
You can choose and execute one of the following commands in PowerShell:
A. Appends the bin directory to your user environment variable `PATH`:
```
[Environment]::SetEnvironmentVariable("Path", [Environment]::GetEnvironmentVariable("Path", "User") + ";C:\Users\runneradmin\AppData\Roaming\Python\Scripts", "User")
```
B. Tries to appends the bin directory to PATH every when you run PowerShell (>=6 recommended):
```
echo 'if (-not (Get-Command poetry -ErrorAction Ignore)) { $env:Path += ";C:\Users\runneradmin\AppData\Roaming\Python\Scripts" }' | Out-File -Append $PROFILE
```
Alternatively, you can call Poetry explicitly with `C:\Users\runneradmin\AppData\Roaming\Python\Scripts\poetry`.
You can test that everything is set up by executing:
`poetry --version`

[johnthagen]

For reviewers, here is a useful StackOverflow answer on the subject: https://stackoverflow.com/a/2571200

[johnthagen]

[Disclaimer, not a Poetry dev]

First off, thank you for making this PR. I think this is an important step to easing new Windows users into Poetry.

My personal feedback is that we should try to keep this simpler. The installer already prints quite a lot of text and having multiple Powershell commands I think could overwhelm the target audience.

If we look at the Unix variant of this for inspiration, it is very simple:

POST_MESSAGE_CONFIGURE_UNIX = """
Add `export PATH="{poetry_home_bin}:$PATH"` to your shell configuration file.
"""

I would recommend we shorten this message to:

POST_MESSAGE_CONFIGURE_WINDOWS = """
Execute the following in Powershell:

[Environment]::SetEnvironmentVariable("Path", [Environment]::GetEnvironmentVariable("Path", "User") + ";{poetry_home_bin}", "User")
"""

[johnthagen]

I tested Option A in this PR in a clean Windows 10 VM, and it worked as expected. 🚀

[tats-u]

Unix OSes have no choice other than editing profile files for their shells, so how to activate Poetry can be kept simple.

In contrast of this, Windows has the built-in manager of environment variables.
PowerShell and CMD have profiles like shells for Unix.
This is why 2 option can be provided for Windows.

[johnthagen]

@neersighted Would you be willing to review this PR? I personally think it would be important before the upcoming 1.2 final release. ❤️