fix: do not use dynamic module in post-script for PowerShell

[lewis-yeung]

Currently, the dynamic module used in a post-script for PowerShell exports a prompt function, which overrides the original. It also uses an environment variable NVS_ORIGINAL_PROMPT to store the entire stringified original prompt function. However, there is a flaw: if the original prompt is defined inside a dynamic module as well, functions defined in that module will become inaccessible for $env:NVS_ORIGINAL_PROMPT, which is invoked by the NVS-defined prompt. This is caused by some "isolation" features of PowerShell modules. Related code:

nvs/lib/auto.js

Lines 120 to 123 in bf45ef0

	'New-Module -Script {',
	' function prompt { . "' + psScriptFile + '" "prompt" }',
	' Export-ModuleMember -Function prompt',
	'} > $null',

Related issue: JanDeDobbeleer/oh-my-posh#2568.

Since NVS is not a tool of prompt customization, it should respect the user's custom prompt function. As an alternative, we can save the original prompt in a global variable when we use nvs auto on to enable the auto-switching so that it can be invoked/restored correctly. Also, we have to store the original $? in another global variable which is accessible for the initialization script of a prompt customization tool and restore the original $global:LASTEXITCODE after the auto-switching done so that the original prompt function can get the correct error code.

❤️ Special thanks to @possum-enjoyer for the help and comments in #253.

[brian-wittenberg-relativity]

I can confirm this is working, as I've been using it for several weeks since I use posh-git and out of the box NVS was breaking my prompt when using nvs auto on.

[jasongin]

Thanks for the contribution! And sorry for the delay in merging.

[jasongin]

Released as nvs v1.7.0.