SimpleDebounce should be a disposable

This should be disposable:

https://github.com/microsoft/vscode-python-environments/blob/baad0daaa2e01f31ba507176e889bdfaf30d4cff/src/common/utils/debounce.ts#L1-L23

`dispose`'s impl should be:

```ts
public dispose() {
if (this.timeout) {
clearTimeout(this.timeout);
}
}
```

Not having this means that if the debouncer's job is done while the `setTimeout` is active, it will still fire again.

An example of this in practice is if the extension is disabled it should immediately dispose everything. Another example is the transient usage in https://github.com/microsoft/vscode-python-environments/pull/952