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:

    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