Implement interpreter support for `deno_task_shell`

[wolfv]

We would like to support deno_task_shell as interpreter for the scripts (as an alternative to bash and cmd.exe).

The big benefit is that it would work well on all platforms incl. Windows.

The downside is that it doesn't support some more complex features such as proper if/else.

[trim21]

the lake of if/else may be not a big issue since it's possible to use if/else in script.content.*.if/else

[wolfv]

Yep, I recently had the same thought!