Support building and serving examples

[pragmatrix]

This is a rudimentary attempt to support building, serving, and watching examples by simply forwarding the --example command line argument to cargo.

The implementation does not look up index.html in the examples/ directory (yet) but nevertheless can be useful even though index.html is loaded from the Cargo.toml manifest directory.

Closes #423

Checklist

• Updated CHANGELOG.md describing pertinent changes.
• Updated README.md with pertinent info (may not always apply).
• Updated site content with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done. If you don't, then Dodd will 🤓.

[github-actions]

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[github-actions]

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[ctron]

@pragmatrix I would love to have this too for some of my projects :) I assume the idea is to run trunk serve --example foo and have trunk serve the examples/foo/main.rs, right?

Maybe it makes sense to add a few todo's in the initial comment of the PR, like the detection of the index.html file. Like that:

• pass --example to build
• detect index.html in example folder