Remove build before launch in launch.json

And update contributing with new instructions.

.vscode/launch.json

@@ -15,7 +15,6 @@
       "outFiles": [
         "${workspaceRoot}/extensions/ql-vscode/out/**/*.js",
       ],
-      "preLaunchTask": "Build",
       "env": {
         // uncomment to allow debugging the language server Java process from a remote java debugger
         // "DEBUG_LANGUAGE_SERVER": "true"
@@ -62,7 +61,6 @@
       "outFiles": [
         "${workspaceRoot}/extensions/ql-vscode/out/**/*.js",
       ],
-      "preLaunchTask": "Build"
     },
     {
       "name": "Launch Integration Tests - Minimal Workspace (vscode-codeql)",
@@ -79,7 +77,6 @@
       "outFiles": [
         "${workspaceRoot}/extensions/ql-vscode/out/**/*.js",
       ],
-      "preLaunchTask": "Build"
     }
   ]
 }