C# syntax highlighting: support string interpolation syntax

Author: dgrunwald

ICSharpCode.AvalonEdit/Highlighting/Resources/CSharp-Mode.xshd

@@ -3,6 +3,7 @@
 	<!-- The named colors 'Comment' and 'String' are used in SharpDevelop to detect if a line is inside a multiline string/comment -->
 	<Color name="Comment" foreground="Green" exampleText="// comment" />
 	<Color name="String" foreground="Blue" exampleText="string text = &quot;Hello, World!&quot;"/>
+	<Color name="StringInterpolation" foreground="Black" exampleText="string text = $&quot;Hello, {name}!&quot;"/>
 	<Color name="Char" foreground="Magenta" exampleText="char linefeed = '\n';"/>
 	<Color name="Preprocessor" foreground="Green" exampleText="#region Title" />
 	<Color name="Punctuation" exampleText="a(b.c);" />
@@ -107,6 +108,18 @@
 			</RuleSet>
 		</Span>
 
+		<Span color="String">
+			<Begin>\$"</Begin>
+			<End>"</End>
+			<RuleSet>
+				<!-- span for escape sequences -->
+				<Span begin="\\" end="."/>
+				<Span begin="\{\{" end=""/>
+				<!-- string interpolation -->
+				<Span begin="{" end="}" color="StringInterpolation" ruleSet=""/>
+			</RuleSet>
+		</Span>
+		
 		<!-- don't highlight "@int" as keyword -->
 		<Rule>
 			@[\w\d_]+