Capacitor could not find the web assets directory

**Description:**

Trying to run on iOS on a blank project but getting the below error. From the docs as I understand it correctly I just need to run that command without any previous commands.

**Steps to Reproduce:**

```
❯ ionic start

Pick a framework! 😁

Please select the JavaScript framework to use for your new app. To bypass this prompt next time, supply a value for the
--type option.

? Framework: Angular

Every great app needs a name! 😍

Please enter the full name of your app. You can change this at any time. To bypass this prompt next time, supply name,
the first argument to ionic start.

? Project name: test

Let's pick the perfect starter template! 💪

Starter templates are ready-to-go Ionic apps that come packed with everything you need to build your app. To bypass this
prompt next time, supply template, the second argument to ionic start.

? Starter template: blank
✔ Preparing directory ./test - done!
✔ Downloading and extracting blank starter - done!
? Integrate your new app with Capacitor to target native iOS and Android? Yes
```

**Output:**

```
❯ ionic capacitor run ios -l --verbose
ionic:lib Terminal info: { ci: false, shell: '/bin/zsh', tty: true, windows: false } +0ms
ionic:lib CLI global options: { _: [ 'capacitor', 'run', 'ios' ], help: null, h: null, verbose: true, quiet: null, interactive: true, color: true, confirm: null, json: null, l: true, project: null, '--': [] } +4ms
ionic:lib:project Project type from config: @ionic/angular (angular) +0ms
ionic:lib:project Project details: { configPath: '/Users/marvinheilemann/Downloads/test/ionic.config.json', errors: [], context: 'app', type: 'angular' } +0ms
ionic Context: { binPath: '/Users/marvinheilemann/.nvm/versions/node/v12.18.0/lib/node_modules/@ionic/cli/bin/ionic', libPath: '/Users/marvinheilemann/.nvm/versions/node/v12.18.0/lib/node_modules/@ionic/cli', execPath: '/Users/marvinheilemann/Downloads/test', version: '6.11.0' } +0ms
> capacitor add ios
[error] Capacitor could not find the web assets directory "/Users/marvinheilemann/Downloads/test/www".
Please create it and make sure it has an index.html file. You can change
the path of this directory in capacitor.config.json (webDir option).
You may need to compile the web assets for your app (typically 'npm run build').
More info: https://capacitorjs.com/docs/basics/building-your-app
[ERROR] An error occurred while running subprocess capacitor.

capacitor add ios exited with exit code 1.

Re-running this command with the --verbose flag may provide more information.
ionic:utils-process onBeforeExit handler: 'process.exit' received +0ms
ionic:utils-process onBeforeExit handler: running 1 functions +1ms
ionic:utils-process processExit: exiting (exit code: 1) +61ms
```

**My `ionic info`:**

```
Ionic:

Ionic CLI : 6.11.0 (/Users/marvinheilemann/.nvm/versions/node/v12.18.0/lib/node_modules/@ionic/cli)
Ionic Framework : @ionic/angular 5.3.1
@angular-devkit/build-angular : 0.901.12
@angular-devkit/schematics : 9.1.12
@angular/cli : 9.1.12
@ionic/angular-toolkit : 2.3.0

Capacitor:

Capacitor CLI : 2.4.0
@capacitor/core : 2.4.0

Utility:

cordova-res : not installed
native-run : not installed

System:

NodeJS : v12.18.0 (/Users/marvinheilemann/.nvm/versions/node/v12.18.0/bin/node)
npm : 6.14.7
OS : macOS Catalina
```

**Other Information:**

Start by reproducing `ionic capacitor run ios -l --verbose` from the blank Angular project created by `ionic start`, then inspect the handoff to `capacitor add ios` and the generated `capacitor.config.json`. Done means a fresh blank project can run the documented command without the missing `www` web-assets error.

由索引模型根据 Issue 内容生成。