Set up snapshot testing for AST output

[sholderbach]

We are still moving fast and breaking things, before we set up super stringent tests maybe let's validate the output of the parser for changes.

I simply took what cargo run would output for the different example files and made those the initial snapshots. Some of them still contain garbage. As soon as we fix those the snapshots can be updated.

• Add way to get AST output for tests
• Use insta and cargo-insta for snapshot testing
• Commit initial snapshots