Deleting SCOOP_BRANCH not switch to master

[Ash258]

Extracted from #3397

Well apparently deleting the SCOOP_BRANCH config variable did not reset the branch to master correctly. Can't seem to reproduce the issue.

Since SCOOP_BRANCH config is default and created always on installation, it is required for functioning. Since we can't forbid users to delete it we need to adjust it in code.

Should be enough to replace get_config SCOOP_BRANCH with get_config SCOOP_BRANCH 'master' to return default master, when config value do not exists.

[Ash258]

We should create some Hashtable with defaults which would be used in codebase.

$CONFIG_DEFAULT = @{
    'SCOOP_BRANCH' = 'master'
    '...LESS_MSI' = $false
    'aria2' = @{
        'split' = 5
    }
}

So we do not need to duplicate code and could easily change it if needed.

Also get_config could return defaults withotu providing it.

So instead of get_config 'SCOOP_BRANCH', which will be changed to get_config 'SCOOP_BRANCH' 'master', you could implement by default get_config 'SCOOP_BRANCH', which would return $CONFIG_DEFAULT['SCOOP_BRANCH'] if none was found.

https://github.com/lukesampson/scoop/blob/aa8513fb3fb7a3ae6311ec5c6333b2f0a6a8dc2e/lib/config.ps1#L44-L49

L46: return $CONFIG_DEFAULT[$name]

[niheaven]

I like the idea. See discussion before: #3242 (comment)