feat(Set-BuildEnvironment): Allow '_' character for VariableNamePrefix parameter

Fixes #139

Author: fourpastmidnight

BuildHelpers/Public/Set-BuildEnvironment.ps1

@@ -87,7 +87,7 @@ function Set-BuildEnvironment {
         [validatescript({ Test-Path $_ -PathType Container })]
         $Path = $PWD.Path,
 
-        [ValidatePattern('\w*')]
+        [ValidatePattern('(\w|_)*')]
         [String]
         $VariableNamePrefix = 'BH',