A couple of small improvements to the custom vue component example

[josephburkhart]

Hi there, this is my first PR for this project and I'm somewhat new to collaborative Open Source development, so please be patient.

Description

This PR modifies the Custom Vue Component example to make it a bit more helpful for developers who need to create custom components but haven't used Vue before.

Motivation

In the example linked above, the Vue component is defined in counter.js. This way of bundling a component in a vanilla JavaScript file is obviously supported by Vue, since every built-in NiceGUI component is defined this way, but the official Vue documentation (and basically every other resource on the web) assumes that developers are writing .vue files, which are supported by additional tooling. I think the example should structure its Vue code in a way that makes it easier for new developers to understand how it works in relation to existing Vue documentation.

Changes

In the new modified example, the python class definition now uses a new file, counter.vue, which re-implements counter.js in the more customary Vue Single-File Component format.

The call to ui.run() has been modified to ensure browser reload on changes to the .js and .vue files, which is important for iterative development on the Vue side.

Testing

These changes do not add any new functionality that needs to be covered by tests. I have run the modified example and ensured that it does not error.

Additional Notes

I am curious if the developers would be interested in adding a second, more advanced custom vue component example. I am in the process of creating a pannable and zoomable image component, which I would be happy to wrap up in a nice example and provide in a separate PR.