Fix examples in HTML mode

[skirtles-code]

Fixes #1634.

Several of the examples don't work correctly if the HTML/SFC switch is set to HTML.

The main problem is that <input /> and <br /> is transformed to <input></input> and <br></br>. For some examples this is harmless and just leads to strange-looking code, but for other examples it breaks the parsing, leading to incorrect rendering or an error.

Rather than fixing the transform I've just removed the unnecessary closing slash from those tags.

There were also a couple of problems with the Circle Drawer demo. These only applied to the Composition API version. Two things used in the template were not being returned from setup. In SFC mode this still worked because the return is stripped out by a transform, but in HTML mode that return needs to return the correct things.

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