fix(scoop-alias): Prevent overwrite existing file when adding alias

[HUMORCE]

Description

Motivation and Context

This feature exists in the tests, but is not implemented.

How Has This Been Tested?

before:

~\scoop
❯ ni .\shims\scoop-xxx.ps1 | Out-Null
~\scoop
❯ scoop alias add xxx 'new alias'
~\scoop
❯ cat .\shims\scoop-xxx.ps1
# Summary:
new alias
~\scoop
❯

after:

~\scoop
❯ ni .\shims\scoop-xxx.ps1 | Out-Null
~\scoop
❯ scoop alias add xxx 'new alias'
File 'scoop-xxx.ps1' already exists in shims directory.
~\scoop
❯ cat .\shims\scoop-xxx.ps1
~\scoop
❯

bin\test.ps1 in powershell 5 and 7, all passed

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.

[niheaven]