Request perms at runtime to connect to net

[taoeffect]

Problem

Need to make sure contract code doesn't try anything funny.

Solution

https://deno.land/manual/runtime/permission_apis

Consider also doing the same for write permissions when <url-or-dir> is a directory that's outside of .

May want to allow net in general case (e.g. to make things like deno deploy smoother), and restrict it when using any of the contract-related commands.

See also comment in src/upload.ts