feat(task): support object notation, remove support for JSDocs

[bartlomieju]

This commit changes three aspects of deno task:

1. Tasks can now be written using object notation like so:

{
  "tasks": {
     "foo": "deno run foo.js",
     "bar": {
        "command": "deno run bar.js"
     }
}

2. Support for comments for tasks is now removed. Comments above tasks will
   no longer be printed when running deno task.
3. Tasks written using object notation can have "description" field that replaces
   support for comments above tasks:

{
  "tasks": {
     "bar": {
        "description": "This is a bar task"
        "command": "deno run bar.js"
     }
}

$ deno task
Available tasks:
- bar
    // This is a bar task
    deno run bar.js

Pulled most of the changes from #26467 to
support "dependencies" in tasks. Additionally some cleanup was performed to make
code easier to read.

[dsherret]

LGTM

[nathanwhit]

LGTM