Pass --extension switch instead of the legacy --extensions

The previous switch was badly named, so this is the better name.

Author: jasonmalinowski

package.json

@@ -1184,6 +1184,7 @@
             "order": 0
           },
           "dotnet.server.extensionPaths": {
+            "scope": "machine-overridable",
             "type": [
               "array",
               null
@@ -1192,7 +1193,7 @@
               "type": "string"
             },
             "default": null,
-            "description": "Override for path to langauge server --extensions argument"
+            "description": "Override for path to langauge server --extension arguments."
           },
           "dotnet.implementType.insertionBehavior": {
             "type": "string",

src/lsptoolshost/roslynLanguageServer.ts

@@ -512,7 +512,7 @@ export class RoslynLanguageServer {
         let args: string[] = [];
 
         for (const extensionPath of extensionPaths) {
-            args.push("--extensions"); // TODO: switch to --extension naming
+            args.push("--extension");
             args.push(extensionPath);
         }