Added the BeIn assertion

Functions/Assertions/BeIn.Tests.ps1

@@ -0,0 +1,18 @@
+Set-StrictMode -Version Latest
+
+InModuleScope Pester {
+    Describe "PesterBeIn" {
+        It "passes if value is in the collection" {
+            Test-PositiveAssertion (PesterBeIn 1 (@(1,2,3)))
+            Test-PositiveAssertion (PesterBeIn 'a' (@(1,'a',3)))
+            1 | Should BeIn @(1,2,3)
+            'a' | Should BeIn @(1,'a',3)
+        }
+        It "fails if value is not in the collection" {
+            Test-NegativeAssertion (PesterBeIn 4 (@(1,2,3)))
+            Test-NegativeAssertion (PesterBeIn 'b' (@(1,'a',3)))
+            4 | Should Not BeIn @(1,2,3)
+            'b' | Should Not BeIn @(1,'a',3)
+        }
+    }
+}

Functions/Assertions/BeIn.ps1

@@ -0,0 +1,16 @@
+
+function PesterBeIn($value, $expectedArrayOfValues) {
+    return [bool]($expectedArrayOfValues -contains $value)
+}
+
+function PesterBeInFailureMessage($value, $expectedArrayOfValues) {
+    if(-not ([bool]($expectedArrayOfValues -contains $value))) {
+        return "Expected: ${value} to be in collection [$($expectedArrayOfValues -join ',')] but was not found."
+    }
+}
+
+function NotPesterBeInFailureMessage($value, $expectedArrayOfValues) {
+    if([bool]($expectedArrayOfValues -contains $value)) {
+        return "Expected: ${value} to not be in collection [$($expectedArrayOfValues -join ',')] but was found."
+    }
+}

en-US/about_Should.help.txt

@@ -36,6 +36,11 @@ LONG DESCRIPTION
 
         $Error.Count | Should BeGreaterThan 0
 
+    BeIn
+        Asserts that a collection of values contain a specific value. Uses PowerShell's -contains operator to confirm.
+
+        1 | Should BeIn @(1,2,3,'a','b','c')
+
     BeLessThan
         Asserts that a number is less than an expected value. Uses PowerShell's -gt operator to compare the two values.