Add C# problem matcher (#2)

* Add c# problem matcher

* ##[add-matcher]

src/setup-dotnet.ts

@@ -1,5 +1,6 @@
 import * as core from '@actions/core';
 import * as installer from './installer';
+import * as path from 'path';
 
 async function run() {
   try {
@@ -14,7 +15,9 @@ async function run() {
     }
 
     // TODO: setup proxy from runner proxy config
-    // TODO: problem matchers registered
+
+    const matchersPath = path.join(__dirname, '..', '.github');
+    console.log(`##[add-matcher]${path.join(matchersPath, 'csc.json')}`);
   } catch (error) {
     core.setFailed(error.message);
   }