refactor(core): Rewrite and separate path-related functions to system.ps1

[niheaven]

Description

Ed. Separate part of isolate PATH to #5840, this PR is for refactoring now.

Use config option use_isolated_path to separate Scoop apps PATH to SCOOP_PATH, and just add %SCOOP_PATH% to $env:Path when install app with env_add_path first time.

Motivation and Context

• Prepare for feat(path): Isolate Scoop apps' PATH #5840

What the commits do:

• refactor(core): Remove unused ensure_robocopy_in_path()
  • The function is used when installing scoop itself, and the install script is moved to https://github.com/ScoopInstaller/Install/blob/master/install.ps1
• refactor(system): Move EnvVar-related functions to system.ps1
  • Move from core.ps1: Publish-Env(), env(), search_in_path(), ensure_in_path(), strip_path(), add_first_in_path(), remove_from_path()
  • Add necessary system.ps1 importing
• refactor(EnvVar): Separate env() to Get-EnvVar() and Set-EnvVar()
  • Copied and modified from https://github.com/ScoopInstaller/Install/blob/master/install.ps1
• refactor(system): Remove search_in_path()
  • Replace with Get-Command().Source
• refactor(system): Refactor PATH-related functions
  • strip_path() -> Find-Path()
  • ensure_in_path()/add_first_in_path() -> Add-Path()
    • ensure_in_path() is only used in shim creation
    • Use -Force to forcedly add a path in the first position as add_first_in_path()
    • Add-Path() supports %
  • remove_from_path() -> Remove-Path()

How Has This Been Tested?

Passed all Scoop tests.

Checklist:

• I have read the Contributing Guide.
• I have ensured that I am targeting the develop branch.
• I have updated the documentation accordingly.
• I have updated the tests accordingly.
• I have added an entry in the CHANGELOG.

[chawyehsu]

I realize there are references of this function in some manifests, but I think this function should not be provided and used outside of the core (though I know there is impossible to hide a function based on the current corebase, all functions are exposed, which I dislike).

What they really want is a way to manipulate $env:PATH-like environment variables, something like Add-PathLikeEnvVar and Remove-PathLikeEnvVar (further abstraction of Add-Path and Remove-Path). This is useful because there are many operations on $env:PATH-like env vars, plus the proposal of $env:SCOOP_PATH is staging. Add-Path and Remove-Path are $env:PATH-specific and may not be much useful if/after $env:SCOOP_PATH is implemented.

[niheaven]

Maybe a refactoring into modules could be done if I have time.

For now, keep the deprecated warning of strip_path, and will remove outside use after this PR goes to main.