CmdPal: Null pattern matching based on is expression rather than overridable operators (#40972)

What the title says. 😄 

Rather than relying on the potentially overloaded `!=` or `==` operators
when checking for null, now we'll use the `is` expression (possibly
combined with the `not` operator) to ensure correct checking. Probably
overkill for many of these classes, but decided to err on the side of
consistency. Would matter more on classes that may be inherited or
extended.

Using `is` and `is not` will provide us a guarantee that no
user-overloaded equality operators (`==`/`!=`) is invoked when a
`expression is null` is evaluated.

In code form, changed all instances of:

```c#
something != null

something == null
```

to:

```c#
something is not null

something is null
```

The one exception was checking null on a `KeyChord`. `KeyChord` is a
struct which is never null so VS will raise an error when trying this
versus just providing a warning when using `keyChord != null`. In
reality, we shouldn't do this check because it can't ever be null. In
the case of a `KeyChord` it **would** be a `KeyChord` equivalent to:

```c#
KeyChord keyChord = new ()
{
    Modifiers = 0,
    Vkey = 0,
    ScanCode = 0
};
```

src/modules/cmdpal/Tests/Microsoft.CmdPal.Ext.Calc.UnitTests/NumberTranslatorTests.cs

@@ -18,8 +18,8 @@ public class NumberTranslatorTests
     public void Create_ThrowError_WhenCalledNullOrEmpty(string sourceCultureName, string targetCultureName)
     {
         // Arrange
-        CultureInfo sourceCulture = sourceCultureName != null ? new CultureInfo(sourceCultureName) : null;
-        CultureInfo targetCulture = targetCultureName != null ? new CultureInfo(targetCultureName) : null;
+        CultureInfo sourceCulture = sourceCultureName is not null ? new CultureInfo(sourceCultureName) : null;
+        CultureInfo targetCulture = targetCultureName is not null ? new CultureInfo(targetCultureName) : null;
 
         // Act
         Assert.ThrowsException<ArgumentNullException>(() => NumberTranslator.Create(sourceCulture, targetCulture));

src/modules/cmdpal/Tests/Microsoft.CmdPal.Ext.TimeDate.UnitTests/TimeAndDateHelperTests.cs

@@ -45,7 +45,7 @@ public class TimeAndDateHelperTests
         var result = TimeAndDateHelper.GetCalendarWeekRule(setting);
 
         // Assert
-        if (valueExpected == null)
+        if (valueExpected is null)
         {
             // falls back to system setting.
             Assert.AreEqual(DateTimeFormatInfo.CurrentInfo.CalendarWeekRule, result);
@@ -72,7 +72,7 @@ public class TimeAndDateHelperTests
         var result = TimeAndDateHelper.GetFirstDayOfWeek(setting);
 
         // Assert
-        if (valueExpected == null)
+        if (valueExpected is null)
         {
             // falls back to system setting.
             Assert.AreEqual(DateTimeFormatInfo.CurrentInfo.FirstDayOfWeek, result);