csharp: Add bracket indents (#22599)

This PR adds an initial `indents.scm` to the C#-extension in order to
support auto-indentation with brackets.

Release Notes:

- N/A

extensions/csharp/languages/csharp/indents.scm

@@ -0,0 +1,3 @@
+(_ "{" "}" @end) @indent
+(_ "[" "]" @end) @indent
+(_ "(" ")" @end) @indent