Different behavior using legacy options

## Problem

Using the deprecated properties `settings` and `extensions` will overwrite/reset all other user settings and not install any other extensions than defined in the property `extensions`.
Using the properties inside the `customizations` property will only **change** the defined settings and install the defined extensions **additionally** to the user extensions.

**Deprecated properties (legacy mode):**

```json
"settings": {},
"extensions": ["ms-python.python"]
```

**Current usage of these properties:**

```json
"customizations": {
"vscode": {
"settings": {},
"extensions": ["ms-python.python"]
}
}
```

The `customizations` property was discussed in #1.

## Proposal

It should be possible to **reset** any user settings/extensions in `customizations` → `vscode` → `settings` / `extensions`.
Maybe add a property `reset`:

```json
"customizations": {
"vscode": {
"reset": {
"settings": true,
"extensions": true,
}
}
}
```