improvement(vscode): Improve tasks

Changed the build task to no longer set isDefault: true. This lets you
set your own default build task from a code workspace.

Also replaced "cd app" in each command by setting the working directory
for the task.

.vscode/tasks.json

@@ -6,11 +6,11 @@
     {
       "label": "Build",
       "type": "shell",
-      "command": "cd app && west build",
-      "group": {
-        "kind": "build",
-        "isDefault": true
+      "command": "west build",
+      "options": {
+        "cwd": "${workspaceFolder}/app"
       },
+      "group": "build",
       "problemMatcher": [
         {
           "base": "$gcc",
@@ -21,13 +21,19 @@
     {
       "label": "Flash",
       "type": "shell",
-      "command": "cd app && west flash",
+      "command": "west flash",
+      "options": {
+        "cwd": "${workspaceFolder}/app"
+      },
       "group": "test"
     },
     {
       "label": "Debug",
       "type": "shell",
-      "command": "cd app && west debug",
+      "command": "west debug",
+      "options": {
+        "cwd": "${workspaceFolder}/app"
+      },
       "group": "test"
     }
   ]