Implementation for PowerShell/PowerShell-RFC#391. A few implementation decisions:
1. Check the package manager list only once per process (**same as CMD**). So, changing the package manager list in registry doesn't affect a PowerShell session that is already running.
2. When detecting user/system PATH value changes, only check new items PREPENDED or APPENDED to the original string (**same as CMD**). The detection is made simple, which is not intended to detect complex changes to the user/system PATH.
3. The PATH update feature is disabled when the `Environment` provider is not available, or when the current session is restricted.
- Both `Environment` provider and `IsSessionRestricted` are checked only once per PowerShell session.
- Use `ConditionalWeakTable` to cache whether the feature is enabled or not for a given session.
- We cannot use `thread-static` variable for the caching because it's possible for Runspace to reuse threads.
Always use the v5.1 `$using:` logic when dealing with any connection type that is not based on the `WSManConnectionInfo`. This allows the caller to use the more advanced `$using` features like variables with special names, having `$using` vars inside sub scopes, and more. `WSMan` based connections might still communicate with older versions but as the oldest supported version is v3 on Server 2012. We don't need to keep the default as v2 (which has been deprecated) to support some of the newer features.
Use the help message on the parameter in the completion tool tip to provide better completion help. This can be utilized by other tools to provide more context for a parameter outside of the current type and parameter name which is quite minimal.
The change causes nested modules of PSReadLine to be loaded to global session state as well.
It's possible to check if we are currently importing the PSReadLine module but the check would depend on the current design of the PSReadLine module, such as how many nested modules it has, its module type, and etc., which is fragile. It's safer to just revert the change.
