Allow debugging via vscode for dungeon puzzler

[gwilymk]

It actually works! Will want to test this and work out how best to support non-linux operating systems too before putting it in the template.

Based on https://felixjones.co.uk/mgba_gdb/vscode.html

• no changelog update needed

[corwinkuiper]

I have CARGO_TARGET_DIR set for subvolume reasons, which I can use with ${env:CARGO_TARGET_DIR}, but I don't think the ${env:CARGO_TARGET_DIR:target} syntax works here. Looks like this won't work out of box for both of us :/

[gwilymk]

I tried for ages to make it optional, or to try and autodiscover it but failed. So I've gone with forcing CARGO_TARGET_DIR to be target/ which sucks but at least it should now work for you