Initial commit for System.Text.Rune (dotnet/coreclr#20935)

This type represents a Unicode scalar value ([ U+0000..U+D7FF ], inclusive; and [ U+E000..U+10FFFF ], inclusive). The primary scenario is for having a consistent representation of Unicode data regardless of the underlying input encoding type, including abstracting away surrogate code points.

Commit migrated from dotnet/coreclr@7fcd8a8

Author: GrabYourPitchforks

src/coreclr/src/System.Private.CoreLib/Resources/Strings.resx

@@ -3652,6 +3652,9 @@
   <data name="Argument_MethodRedefined" xml:space="preserve">
     <value>Method has been already defined.</value>
   </data>
+  <data name="Argument_CannotExtractScalar" xml:space="preserve">
+    <value>Cannot extract a Unicode scalar value from the specified index in the input.</value>
+  </data>
   <data name="Argument_CannotParsePrecision" xml:space="preserve">
     <value>Characters following the format symbol must be a number of {0} or less.</value>
   </data>

src/coreclr/src/System.Private.CoreLib/src/System/ThrowHelper.cs

@@ -76,6 +76,11 @@ internal static void ThrowArgumentException_OverlapAlignmentMismatch()
             throw new ArgumentException(SR.Argument_OverlapAlignmentMismatch);
         }
 
+        internal static void ThrowArgumentException_CannotExtractScalar(ExceptionArgument argument)
+        {
+            throw GetArgumentException(ExceptionResource.Argument_CannotExtractScalar, argument);
+        }
+
         internal static void ThrowArgumentOutOfRange_IndexException()
         {
             throw GetArgumentOutOfRangeException(ExceptionArgument.index,
@@ -490,6 +495,7 @@ internal enum ExceptionArgument
         pHandle,
         values,
         task,
+        ch,
         s,
         input,
         pointer,
@@ -528,6 +534,7 @@ internal enum ExceptionResource
         ArgumentOutOfRange_Index,
         Argument_InvalidOffLen,
         Argument_ItemNotExist,
+        Argument_CannotExtractScalar,
         ArgumentOutOfRange_Count,
         ArgumentOutOfRange_InvalidThreshold,
         ArgumentOutOfRange_ListInsert,

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -651,9 +651,12 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\EncodingProvider.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\Latin1Encoding.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\NormalizationForm.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Text\Rune.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\StringBuilder.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\StringBuilder.Debug.cs" Condition="'$(Configuration)' == 'Debug'" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Text\UnicodeDebug.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\UnicodeEncoding.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Text\UnicodeUtility.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\Utf16Utility.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\UTF32Encoding.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\UTF7Encoding.cs" />