Before, the Vue `<SvgIcon>` always outputs DOM nodes like:
```html
<span class="outer-class">
<svg class="class-name-defined" ...></svg>
</span>
```
The `span` is redundant and I guess such layout and the inconsistent
`class/class-name` attributes would cause bugs sooner or later.
This PR makes the `<SvgIcon>` clear, and it's faster than before,
because it doesn't need to parse the whole SVG string.
Before:
<details>

</details>
After:

---------
Co-authored-by: silverwind <me@silverwind.io>
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|---|---|---|
| .. | ||
| components | ||
| features | ||
| markup | ||
| modules | ||
| standalone | ||
| test | ||
| utils | ||
| webcomponents | ||
| bootstrap.js | ||
| index.js | ||
| jquery.js | ||
| serviceworker.js | ||
| svg.js | ||
| svg.test.js | ||
| utils.js | ||
| utils.test.js | ||