Add shellenv template for PowerShell

[jpawlowski]

This is a more simple way to support running the brew command inside of a PowerShell session that is not a child process of a regular Unix shell session (e.g. where existing environment variables are not inherited). It is useful in case one wants to have a dedicated shell profile setup for PowerShell in Apple Terminal, iTerm, or other terminal emulators.

It is as simple as running brew shellenv | Invoke-Expression at the beginning of a PowerShell session or by adding the same command to the PowerShell $PROFILE file.

To automate this, the following command can be run once to finish the setup inside of a PowerShell session:

Add-Content -Path $PROFILE -Value '$(if (Test-Path -PathType Leaf /opt/homebrew/bin/brew) {/opt/homebrew/bin/brew shellenv} elseif (Test-Path -PathType Leaf /usr/local/bin/brew) {/usr/local/bin/brew shellenv}) | Invoke-Expression -ErrorAction SilentlyContinue'
$(if (Test-Path -PathType Leaf /opt/homebrew/bin/brew) {/opt/homebrew/bin/brew shellenv} elseif (Test-Path -PathType Leaf /usr/local/bin/brew) {/usr/local/bin/brew shellenv}) | Invoke-Expression -ErrorAction SilentlyContinue

[MikeMcQuaid]

Thanks so much for your first contribution and sponsorship! Without people like you submitting PRs we couldn't run this project. You rock, @jpawlowski!