CSharp: Add 'warnaserror:...' to v142 flag table for VS

This allows listing the warnings that must be flagged as errors.
For example:

    target_compile_options(proj PRIVATE "/warnaserror:1998,4014")

Templates/MSBuild/FlagTables/v142_CSharp.json

@@ -270,6 +270,17 @@
     "value": "",
     "flags": []
   },
+  {
+    "name":  "WarningsAsErrors",
+    "switch": "warnaserror:",
+    "comment": "",
+    "value": "",
+    "flags": [
+      "UserValue",
+      "UserRequired",
+      "CommaAppendable"
+    ]
+  },
   {
     "name":  "WarningLevel",
     "switch": "warn:0",